Incremental refresh of materialized views with joins and aggregates after arbitrary DML operations to multiple tables6882993Abstract A method is provided for incrementally refreshing a materialized view after multiple operations on a row of a base table of the materialized view, by determining an equivalent operation for the multiple operations and refreshing the materialized view according to the equivalent operation. The method is applicable to a materialized view based on multiple base tables on which multiple operations have been performed. The step of determining the equivalent operation can include identifying rows for which an earliest operation is a DELETE operation, or rows for which a latest operation is an INSERT operation, or a combination of the two. The step of refreshing the materialized view includes performing an inverse operation of the equivalent operation to determine a pre-update state of the row, and refreshing the materialized view based on the pre-update state. Additional embodiments are provided which enhance the performance of materialized view refresh queries. Claims What is claimed is: Description FIELD OF THE INVENTION
Column Name Type
M_ROW$$ ROWID
SEQUENCE$$ NUMBER
DMLTYPE$$ VARCHAR2(1)
OLD_NEW$$ VARCHAR2(1)
The M_ROW$$ column captures the ROWID for the modified row of the base table. The SEQUENCE$$ column is used to determine the order of operations on each row. As each change is being logged, a corresponding row is added to the MLOG$ T table. Within each MLOG$_T row, the SEQUENCE$$ column is assigned a monotonically increasing unique value, or sequence number. Thus, each operation of a given ROWID will get a unique sequence number and later operations will have a larger sequence number than earlier operations. The DMLTYPE$$ column indicates the type of row operation that has occurred and can have values `I` (INSERT), `D` (DELETE), and `U` (UPDATE). The OLD_NEW$$ column indicates whether the logged row corresponds to the state of the row before or after the operation. In the case of INSERT operations, OLD_NEW$$ has the value `N` indicating a new value. In the case of DELETE operations, OLD_NEW$$ has the value `O` indicating an old value. For UPDATE operations, both the old and new values are logged in two separate rows with OLD_NEW$$ values `O` and `N` respectively. Also, for updated rows, the pre-update version of the row has a sequence number less than the sequence number of the post-update version. Thus, effectively, an UPDATE on a row is treated as a DELETE of the pre-update row followed by an INSERT of the post-update row. Another mechanism for logging base table operations relates to a type of INSERT called DIRECT PATH INSERT, which is a fast bulk insert which inserts contiguous ranges of ROWID values. Inserts made through the DIRECT PATH mechanism are logged into a direct load log, which only maintains the ROWID ranges inserted but does not log the individual column values. The direct load log is a system-wide log named ALL_SUMDELTA that does not log information on a per table basis but logs any rows corresponding to bulk inserts to any table, and has the following or comparable columns.
Column Name Type
TABLEOBJ# NUMBER
LOWROWID ROWID
HIGHROWID ROWID
SEQUENCE NUMBER
Each table is identified by the TABLEOBJ#. The LOWROWID and HIGHROWID columns store the bounds of the ROWID ranges for the bulk INSERT. Each ROWID range is assigned a sequence number, using the same sequence as the materialized view log. The sequence number can be used to determine the ordering of various direct loads and also between direct loads and other conventional, or independent, base table operations. It is acceptable to assign the same sequence number to all of the rows corresponding to a ROWID range in ALL-SUMDELTA because it is still possible to find the order of operations on a given row. Computation of Delta Rows and the Pre-Update State of a Base Table In this section, the following schema with two tables is used as an example, but the techniques described can be generalized to any number of tables. CREATE TABLE F (a integer, b integer, c integer); CREATE TABLE T (a integer, b integer, c integer). Each of the tables has a materialized view log (MLOG$_F and MLOG$_T), such as described above. Furthermore, a materialized view represented by the following query is used for illustrative purposes. SELECT t.b, t.c, COUNT(*) as f_cstar FROM F f, T t WHERE f.a=t.a GROUP BY t.b, t.c. As previously described, if more than one base table associated with a materialized view is operated upon, then a refresh of the materialized view involves computation of the pre-update state of at least one of the base tables. Computation of the pre-update state for a base table is straightforward when the operations performed on that base table consist of INSERTs only or DELETEs only. For example, if the operations consist of a series of INSERTs only, then the pre-update state can be computed by removing the inserted rows from the known post-update state of the table. In the absence of any DIRECT PATH INSERTs, the following query could be used to identify the delta rows, or inserted rows, for table T from the corresponding MV log: INS_DELTA_T: SELECT L.m_row$$ rid, L.a, L.b, L.c FROM mlog$_T L WHERE L.old_new$$=`N`. The pre-update state of rows for table T can consequently be determined by the following query: PRE_T: SELECT* FROM T WHERE ROWID NOT IN (SELECT rid FROM INS_DELTA_T). For another example, if the INSERTs to the base table used only the DIRECT PATH for bulk INSERTs, the following queries provide the delta rows and the pre-update state, respectively, of the table T from the ALL_SUMDELTA log: INS_DELTA_T: SELECT t.rowid rid, t.a, t.b, t.c, t.d FROM T t, ALL_SUMDELTA I WHERE (t.rowid between l.lowrowid and l.highrowid) AND (l.obj#=<tableobj# of T>); PRE_T: SELECT t.* FROM Tt WHERE t.rowid NOT IN (SELECT rid FROM INS_DELTA_T). In the presence of both DIRECT PATH and conventional (or independent) row INSERTs, the INS_DELTA_T would consist of a UNION ALL of each INS_DELTA_T query presented above, while the definition of PRE_T is as presented in the INSERT only example. If the base table operations consist of a series of DELETEs only, the pre-update state of the table can be computed by adding to the table the rows that were deleted. The following expression could be used to identify the delta rows for the DELETE only case: DEL_DELTA_T: SELECT L.a, L.b, L.c FROM mlog$_tL WHERE L.old_new$$=`O`; The pre-update state of rows for table T can be determined by the following query, PRE_T: SELECT * FROM T UNION ALL SELECT * FROM DEL_DELTA_T Once delta rows are computed, the delta for the entire base table can be computed by the following query: DELTA_T: (Expression 4) (SELECT a, b, c, `D` as dml FROM DEL_DELTA_T) UNION ALL (SELECT a, b, c, `I` as dml FROM INS_DELTA_T); where the marker `D` or `I` in the delta summary above is used to tag the column in the aggregate with +(-) sign if the row is inserted (deleted). Hence, the pre-update state of a base table T, which can be used to refresh the materialized view, can be provided by the following query: PRE_T: (Expression 5) (SELECT * FROM T WHERE ROWID NOT IN (SELECT RID FROM INS_DELTA_T)) UNION ALL (SELECT * FROM DEL_DELTA_T) Determining an Equivalent Operation for Multiple Operations To a Row The previous examples involved cases in which base tables were changed by INSERT operations only or DELETE operations only. To compute a pre-update state of a row of a base table that has experienced an arbitrary combination of operations, such as INSERT, DELETE, and UPDATE, the operations upon a particular row are represented by an equivalent operation.
TABLE 1
First DML Last DML Equivalent DML
Insert Insert Insert V.sub.n
Insert Delete NOP
Insert Update Insert V.sub.n
Delete Insert Delete V.sub.o, Insert V.sub.n
Delete Delete Delete V.sub.o
Delete Update Delete V.sub.o, Insert V.sub.n
Update Insert Delete V.sub.o, Insert V.sub.n
Update Delete Delete V.sub.o
Update Update Delete V.sub.o, Insert V.sub.n
Table 1 is a table depicting equivalent operations for a series of operations, with "DML" representing a base table row operation. For any particular row of a base table, if the old or original value (i.e., the value before the first operation on that row and column; hereinafter V.sub.o) of the column that has been changed is known, and the new or final value (i.e., the value after the last operation on that row and column; hereinafter V.sub.n) is known, all of the operations on that row can be represented by an equivalent operation or operations as depicted in Table 1 because all intermediate operations are considered inconsequential. For example, if a row is inserted into a base table and subsequently updated numerous times, this series of operations can be equivalently represented as a single insert of the last updated value such that the equivalent operation is "Insert V.sub.n " (as illustrated in the shaded row of Table 1). In another example, if a row is updated numerous times before being deleted, the equivalent operation with respect to the materialized view is "Delete V.sub.0 ". "NOP" represents "No Operation." Thus, Table 1 enumerates the effect on the materialized view, represented as an equivalent operation, for all combinations of first and last operations to a particular row of a base table. For any given row, there are at most two operations that define or represent the equivalent operation, "Delete V.sub.o " and "Insert V.sub.n ". Upon determination of the equivalent operation for one or more operations on a row of a base table, the pre-update state of the base table can be computed according to Expression 5, and thus, the materialized view can be refreshed based on Expression 1. The computation of the equivalent operation for a given row relies on the ability to determine the first and last operation on the row. A method for achieving this includes assigning each operation in the MV logs a monotonically increasing sequence number, such as the values automatically inserted into the SEQUENCE$$ column described above. As such, the first operation on any row is the one that has the least sequence number (MIN(sequence$$)) and the last operation on any row is the one that has the largest sequence number (MAX(sequence$$). In addition, the OLD_NEWS$ column of the MV log is called upon for use in conjunction with the SEQUENCE$$ column to determine rows on which the latest, or last, operation was an INSERT operation and rows on which the earliest, or first, operation was a delete operation (e.g., a strict DELETE or an UPDATE). As described, an UPDATE operation is treated as a DELETE followed by an INSERT. Hence, the following queries are examples of expressions that can be used to compute the delta rows, from the MV log only (thus, deltas due to independent, not bulk, operations), for the DELETE and INSERT deltas, respectively. The following query finds the rows for which the earliest operation was a DELETE operation. DEL_DELTA_T: (Expression 6) SELECT L.m_row$$ rid, L.a, L.b, L.c, `D` as marker FROM mlog$_t L WHERE L.old_new$$=`O` AND L.sequence$$ IN (SELECT MIN(sequence$$) FROM mlog$_t GROUP BY m_row$$). The following query finds the rows for which the latest operation was an INSERT operation. INS_DELTA_T: (Expression 7) SELECT L.m rows$$ rid, L.a, L.b, L.c, T as marker FROM mlog$_t L WHERE AND L.old_new$$=`N` AND L.sequence$$ IN (SELECT MAX(sequence$$) FROM mlog$_t GROUP BY m_row$$). These expressions can be utilized in the expressions for the pre-update state presented above, as in, for example, Expression 5. In a case where bulk INSERTs have been enacted through a DIRECT PATH operation, in addition to conventional operations, the bulk inserts (i.e., direct loads) are logged into the direct load log as discussed above. The direct load log also has a sequence number for each ROWID range inserted, using the same sequence as the materialized view log, thus the relative position of the direct load to the conventional operations is determinable. Note that the sequence numbers must be unique only for each ROWID within a base table and each ROWID range has distinct ROWDs, and so all of the rows in a direct load range can share the same sequence number.
TABLE 2
First DML Last DML Equivalent DML
Insert Insert Insert V.sub.n
Insert Delete NOP
Insert Update Insert V.sub.n
Insert Direct Load Insert V.sub.t
Delete Insert Delete V.sub.o, Insert V.sub.n
Delete Delete Delete V.sub.o
Delete Update Delete V.sub.o, Insert V.sub.n
Delete Direct Load Insert V.sub.t
Update Insert Delete V.sub.o, Insert V.sub.n
Update Delete Delete V
Update Update Delete V.sub.o, Insert V.sub.n
Update Direct load Insert V.sub.t
Direct Load Insert Insert V.sub.n
Direct Load Delete NOP
Direct Load Update Insert V.sub.n
Direct Load Direct Load Insert V.sub.t
Table 2 is an extension table to Table 1, depicting equivalent operations for a series of operations on a row. Table 2 includes the series of operations in which either the first or last operation is from a direct load. Again, V.sub.o and V.sub.n refer to the initial values before the first operation and final values after the last operation, respectively, from the MV log. V.sub.t refers to the final values, or post-update values, of a row after the last direct load operation. Due to the nature of the ALL_SUMDELTA log, the values of the row at the time of the direct load are not available, only the ROWID value is available. If the direct load is the last operation on the table, then the values for the rows may be obtained from the base table itself. The state of the row resulting from the direct load is not available if another operation further updated the row, thus, only the ROWID of the rows operated upon is available. However, as presented in Table 2, the only values needed to compute an equivalent operation when direct load is not the last operation can be obtained from the MV log. Therefore, the problem can be reduced to determining whether a direct load occurred as the first operation to the row, the last operation to the row, or as an intermediate operation to the row. Determining the sequence of operations on a row can be determined by combining the MV log (MLOG$_T) and the direct load log (ALL_SUMDELTA), as in the following expression. This expression selects all of the rows from the MV log and all of the rows from a join of the direct load log to the base table, and unions the results. COMBINED_LOG T: (Expression 8) (SELECT L.M_ROW$$ RIDS$, L.a, L.b, L.c, L.old_new$$, L.sequence$$ seq$$ FROM mlog$_t L) UNION ALL (SELECT T.rowid RID$$, T.a, T.b, T.c, `N` old_new$$, s.sequence seq$$ FROM T, ALL_SUMDELTA s WHERE s.TABLEOBJ#=<tableobj# of T> AND T.rowid BETWEEN s.LOWROWID AND s.HIGHROWID) Note that the join operation is a range join between the base table's ROWIDs and the bound values of the ROWID range in the direct load log. Further note that the old_new$$ value for the direct load log is always `N`, indicating INSERTs. The combined log can be queried as if it was an MV log (mlog$t), which is described below in Expression 9. EXAMPLE The materialized view incremental refresh techniques described herein shall now be described with reference to an example illustrated in FIGS. 5a, 5b, 5c, 5d, 6a, and 6b. FIG. 5a illustrates the state of tables 502a, 504a, and 506a prior to a series of operations thereupon, at time to. Those tables shall be respectively referred to herein as PRE_TIME 502a, PRE_LOCATION 504a, and PRE_PRODUCT_SALES 506a. FIG. 5b illustrates the state of the tables 502b, 504b, and 506b subsequent to a first operation on tables 502a and 506a, at time t.sub.0. Specifically, one row was deleted (ROWID 1) and another row (ROWID 3) has been inserted in the TIME and the _PRODUCT_SALES tables. FIG. 5c illustrates the state of the tables 502c, 504c, and 506c subsequent to second operations on tables 502b and 506b, at time t.sub.2. Specifically, two rows (ROWID 2 and ROWID 3) were updated, and one row was inserted (ROWID 1) in the _TIME and the _PRODUCT_SALES tables. FIG. 5d illustrates the state of the tables 502d, 504d, and 506d subsequent to third operations on tables 502c and 506c, at time t.sub.3. Specifically, one row was deleted (ROWID 2), and two rows (ROWID 1 and ROWID 3) were updated in the _TIME and _PRODUCT_SALES tables. Tables 502d, 504d, and 506d represent the post-update state of the base tables, i.e., POST_TIME, POST_LOCATION, and POST_PRODUCT_SALES. As can be appreciated, an arbitrary mix of conventional operations has been performed on multiple tables, i.e., the TIME 502a and the PRODUCT_SALES 506a tables. The state of the tables 502a, 504a, and 506a represents the state upon which the materialized view will be refreshed. FIG. 6a illustrates a relevant portion of the materialized view log (MLOG$_TIME) for table POST_TIME 502d, which describes the sequence of operations performed on the TIME table, as depicted in FIGS. 5b-5d. As described above, the computation of the equivalent operation for a given row relies on the ability to determine the first and last operation on the row. Therefore, with respect to ROWID 1, the MLOG$_TIME table shows, based on the sequence number SEQ$$, that the first operation (SEQ$$=0) on this row was a DELETE operation, and the last operation (SEQ$$=3) on this row was an UPDATE operation, which is represented as a DELETE followed by an INSERT. This could be determined by applying Expressions 6 and 7 (or Expression 9 for performance enhancement) to MLOG$_TIME, thus, finding the row with minimum sequence number in the SEQ$$ field and an "0" in the OLD_NEW$$ field, indicated as row 602 in FIG. 6a, and the row with the maximum sequence number in the SEQ$$ field and an "N" in the OLD_NEW$$ field, indicated as 604 in FIG. 6a. Referring to Table 1 shows that an equivalent operation for a series of operations that starts with a DELETE and ends with an UPDATE is "DELETE V.sub.o, INSERT V.sub.n." Reference to row 602 shows that V. for ROWID 1 is "JAN 1980," indicated as data 606, and reference to row 604 shows that V.sub.n for ROWID 1 is "DEC 1979," indicated as data 608. Consequently, the DELTA_TIME pseudo-state for ROWID 1 can be represented as "Delete JAN 1980, Insert DEC 1979." Recall that in this example, the pre-update state of the TIME table (PRE_TIME 502a) is an unknown. In order to determine the pre-update state, an inverse operation of the equivalent operation is applied to the post-update state of the TIME table (POST_TIME 502d), a known state. Expression 5 (or Expression 9 for performance enhancement) can be used to compute the pre-update state of the TIME table. Simply stated, to determine PRE_TIME 502a, apply a DELETE Vn and an INSERT V. to POST_TIME 502d. That is, delete "DEC 1979" and insert "JAN 1980" which produces the table shown as 502a in FIG. 5a. A similar process as described above is performed on ROWID 2 and ROWID 3 of the TIME table, which would show that the first operation (SEQ$$=0) on ROWID 2 is represented as a DELETE operation (extracted from the UPDATE operation), and the last operation (SEQ$$=2) on this row was a DELETE operation; and the first operation (SEQ$$=0) on ROWID 3 was an INSERT operation, and the last operation (SEQ$$=4) on this row is represented as a DELETE operation (extracted from the UPDATE operation). Note from tables 502a-d that, in fact, ROWID 3 experienced a series of operations: INSERT, UPDATE, UPDATE. Hence, the equivalent operation for ROWID 2 is, as shown in Table 1, "Delete V.sub.o." V. for ROWID 2 is shown as data 610, "FEB 1980," which is inserted into POST_TIME 502d to arrive at the pre-update state of the row, as depicted in PRE_TIME 502a. The equivalent operation for ROWID 3 is, as shown in Table 1, "Insert V.sub.n." V.sub.n for ROWID 3 is shown as data 612, "JAN 1980," which is deleted from POST_TIME 502d to arrive at the pre-update state of the row, as depicted in PRE TIME 502a. FIG. 6b illustrates a relevant portion of the materialized view log (MLOG$_PRODUCT_SALES) for table POST_PRODUCT_SALES 506d, which describes the sequence of operations performed on the PRODUCT_SALES table, as depicted in FIGS. 5b-5d. The process described above for the TIME table is applied similarly to the PRODUCT_SALES table, in its various states represented as 506a-d. That is, the DELTA pseudo-state and the pre-update state is computed for the PRODUCT_SALES table, addressing each row that has been operated upon subsequent to the last materialized view refresh. Note from Expressions 1-3, that for a materialized view depending on n base tables, n DELTA pseudo-states but only n-1 pre-update states need be computed to refresh the materialized view. Thus, in this example, computations on the PRODUCT_SALES table could stop after computing the DELTA pseudo-state, or essentially after determining the equivalent operation. Once the necessary DELTA and/or pre-update states have been computed for each base table of the materialized view that has experienced operations since the last refresh, Expressions 1, 2, or 3 can be applied, utilizing the last known state of the materialized view, to refresh the materialized view according to the arbitrary sequences of operation on the multiple base tables. Consequently, the refreshed materialized view will contain the new aggregated values resulting from the base table operations. Method of Refreshing a Materialized View FIG. 7 is a flow diagram illustrating a method for refreshing a materialized view, according to an embodiment of the invention. At step 702, an equivalent operation for a plurality of operations on a row of a base table of a materialized view is determined. This step is exemplified in the Example above. At step 704, the materialized view is refreshed based on the equivalent operation determined in the previous step. Various techniques may be used to incrementally refresh a materialized view, some of which are known in the art. One technique is described in U.S. Pat. No. 6,205,451, issued Mar. 20, 2001, and entitled "METHOD AND APPARATUS FOR INCREMENTAL REFRESH OF SUMMARY TABLES IN A DATABASE MANAGEMENT SYSTEM," the contents of which are incorporated herein by reference. FIG. 8 is a flow diagram illustrating a method for refreshing a materialized view, according to an embodiment of the invention. If a plurality of operations on a row of a base table of a materialized view are described in a materialized view log and a direct load log, then at step 802, the materialized view log and the direct load log are combined. This step can be performed by executing Expression 8, or a similar query. At step 804, an earliest delete operation and/or a latest insert operation, within the plurality of operations, is identified. This step can be performed by executing the expression corresponding to DLT_T$$ in Expression 9, or a similar query. At step 806, an equivalent operation for the plurality of operations is determined according to the earliest delete and/or latest insert operations. An inverse operation of the equivalent operation is performed on the row to determine a pre-update state of the row, at step 808, which can be performed by executing Expression 5, or the expression corresponding to main query block of Expression 9, or a similar query. At step 810, the materialized view is refreshed based on the pre-update state of the row. Various computational performance-enhancing, or optimization methods, which represent embodiments of the present invention, are described below. Optimization is the process of choosing the most efficient way to phrase and execute a SQL statement. This is an important step in the processing of any Data Manipulation Language statement (SELECT, INSERT, UPDATE, or DELETE). There may be many different ways for a database management system to execute such a statement, varying, for example, which tables or indexes are accessed in which order. The procedure used to execute a statement can greatly affect how quickly the statement executes. An optimizer program is typically used to choose an execution process that it determines to be the most efficient, and considers a number of factors in making a choice among its alternatives. Improving Performance Through Partitioning and Materialization The computational performance of Expressions 6 and 7 is increased significantly by combining them into a single query, and thus, determining both the insert delta and delete delta together. An example query that combines the insert and delete deltas, that materializes the combined delta statement into a temporary table, that computes the pre-update state (all in the main query block), and which utilizes analytic functions to provide enhanced performance, follows:
WITH DLT_T$$ AS (Expression 9)
(SELECT d.RID$$, a, b, c, OLD_NEW$$
FROM (SELECT t.*
MIN(t.SEQ$$) OVER (PARTITION BY t.RID$$) MINSEQ#,
MAX(t.SEQ$$) OVER (PARTITION BY t.RID$$) MAXSEQ#
FROM (SELECT M_ROW$$ RID$$, a, b, c, SEQUENCE$$ SEQ$$,
OLD_NEW$$
FROM mlog$_t) t
) d
WHERE (d.OLD_NEW$$ = `O` AND (d.SEQ$$ = d.MINSEQ#)) OR
(d.OLD_NEW$$ = `N` AND (d.SEQ$$ = d.MAXSEQ#)))
(SELECT rowid, a, b, c [INS_DELTA]
FROM t
WHERE rowid NOT IN
(SELECT RID$$
FROM
DLT_T$$
WHERE OLD_NEW$$= `N`)
)
UNION ALL
(SELECT RID$$, a, b, c [DEL_DELTA]
FROM DLT_T$$
WHERE OLDNEW$$ = `O`
).
The temporarily materialized query for DLT_T$$ computes the MIN and the MAX sequence number for each rowid and selects the rows from MLOG$_T corresponding to the earliest DELETE operation or the latest INSERT operation. This is used to compute the pre-update state in the main query block (i.e., the INS_DELTA and the DEL_DELTA branches of the UNION ALL). The use of analytic functions make the query much simpler by avoiding self-joins and "Group by" aggregation. The expression for DLT_T$$ is logically equivalent to a "UNION ALL" of Expressions 6 and 7, but more efficient. In addition, the combined delta statement is materialized into a temporary table using a WITH clause so that it can efficiently be reused multiple times. Running multiple queries is relatively slow because tables (MLOG$_T, in this case) are accessed multiple times for each returned row. It is faster to cache the values from a complex query in a temporary table, then run the queries against the temporary table. A temporary table provides a mechanism for running a computation once, and caching the result in later queries and joins. Furthermore, in complex queries that process the same subquery multiple times, the WITH clause lets the subquery be factored out, given a name, and then referenced multiple times within the original complex query. This technique lets the optimizer program choose how to deal with the subquery results, i.e., whether to create a temporary table or treat it as an inline view. There is a case that necessitates special handling, that is when a row was inserted using a direct load as the first operation on that row, and after some changes the row was finally deleted from the table. From Table 2, it is shown that the equivalent operation for this case is a NOP (No Operation). However, using Expression 9 with the combined log determined from Expression 8, the row would appear as a delete in the MV log but would not appear in the join between the direct load log and the base table because the ROWID is no longer present in the base table. Without special handling, the equivalent operation would be considered a DELETE, which would yield incorrect results. To handle this case, in one embodiment, the case is detected, and the affected rows are added as an INSERT to the pre-update state query to compensate for the extra DELETE. Note that this extra statement is only added if DELETEs have occurred. As long as the row is initially or finally present in the base table, the equivalent operation is determinable using Expression 9. Such a special case is detected by determining whether there were any direct loads with a subsequent DELETE operation. This is partially accomplished by checking for row operations with a sequence number (SEQUENCE$$) larger than direct load sequence number for the same ROWID. This can be accomplished with the following query: SPECIAL DELETES SELECT L.rowid, L.a, L.b, L.c, `N` OLD_NEW$$ FROM mlog$_t L, ALL_SUMDELTA s WHERE s.TABLEOBJ#=<tableobj# of T> AND L.m_row$$ BETWEEN s.LOWROWID AND s.HIGHROWID AND L.SEQUENCE$$>s. SEQUENCE AND L.dmltype$$=`D` AND L.m_row$$ between `RID1` AND `RID2`; For performance reasons, in such a query in which the MV log is joined with the direct load log, restricting the rows from the MV log is advantageous. In one embodiment, the rows of the MV log are restricted by selecting only DELETEs. Further, ranges of ROWIDs on which direct loads were performed are determined by finding the minimum LOWROWID and the maximum HIGHROWID from the direct load log. This results in an efficient query that can be effectively computed using parallelization. An efficient execution plan for the SPECIAL DELETES query could use a SORT MERGE join on MLOG$_T and ALL_SUMDELTA. In parallel, ALL_SUMDELTA could be broadcasted and MLOG$_T distributed by round robin on M_ROW$$. Another method for improving the performance of the handling of the special case involves transforming non-equi joins into equi-joins. This method, referred to as band join, is performed by doing a one-to-one mapping of ROWIDs to numbers such that for any two ROWIDs (RID1 and RID2) and their mappings (N1 and N2), RID1<RID2 implies N1<N2. Next, the following is determined: (1) the minimum number and maximum number mapped to in the ALL_SUMDELTA table; and (2) the largest range of mapped numbers in the ALL_SUMDELTA table, which can be determined with the following query. SELECT MIN(f(LOWROWID)) min_rid, MAX(f(HIGHROWID)) max_rid, MAX(f(HIGHROWID)-f(LOWROWID)) max_range FROM ALL_SUMDELTA; where f is the one-to-one mapping of ROWIDs to numbers. Next, the complete range min_rid to max_rid is split into blocks of size equal to max_range. This can be performed using the following function. MAP(x)=((x-min.sub.-- rid)DIV(max.sub.-- range)) Each row of ALL_SUMDELTA is mapped using the f(LOWROWID) as the argument of the MAP function. The SPECIAL DELTES query can be rewritten using equi-join operations, which can use HASH JOIN between MLOG$_T and ALL_SUMDELTA, which is a more efficient computation due to avoiding a sort operation. Furthermore, in parallel, both ALL_SUMDELTA and MLOG$_T can be distributed by hashing on the respective equi-join keys, which is a more efficient computation than broadcasting of ALL_SUMDELTA. Improving Performance Through Cardinality Hints The MV log (MLOG$_T) and direct load log (ALL_SUMDELTA) tables are typically not analyzed by the user and in the absence of cardinality estimates, the execution plans can deviate significantly from the most efficient plan. The term cardinality refers to the number of cardinal, or basic, items in a set. In a table, such as a database table, the number of rows is used to define the cardinality of the table. To assist in efficiently refreshing a materialized view, a concept of table cardinality is used to indicate in the refresh expression, the number of rows in a base table that have been operated upon and thus, the number of rows that are considered during the refresh process. Passing a hint to the optimizer forces it to use an optimal execution plan. In one embodiment, the cardinalities are derived from the MV log or the direct load log, by issuing COUNT(*) queries before generating the refresh expressions and by adding appropriate hints into the select list of the delta/pre-update expressions. In this embodiment, a cardinality for inserted rows is determined from the MV log, a cardinality for deleted rows is determined from the MV log, and a cardinality for bulk inserted rows is determined from the direct load log. A query that can be issued to collect the cardinality of rows from the MV log is: SELECT OLD_NEW$$, COUNT(*) cnt FROM MLOG$_T GROUP BY OLD_NEW$$. Similarly, the cardinality of rows coming from the direct load log can be estimated by the following query: SELECT SUM(BLOCK_NUM(HIGHROWID)-BLOCK_NUM(LOWROWID)+1)*AVG_ROWS_PER_BLK cnt, COUNT(*) cnt_star FROM ALL_SUMDELTA. The cardinalities determined above are passed as hints to the view refresh expressions. For example, an expression for determining the pre-update state of a table can take the form as follows:
WITH DLT_T$$ AS (Expression 10)
(SELECT d.RID$$, a, b, c, OLD_NEW$$
FROM (SELECT t.*
MIN(t.SEQ$$) OVER (PARTITION BY t.RID$$) MINSEQ#,
MAX(t.SEQ$$) OVER (PARTITION BY t.RID$$) MAXSEQ#
FROM (COMBINED_LOG_T
UNION ALL
SPECIAL_DELETES) t
) d
WHERE (d.OLD_NEW$$ = `O` AND (d.SEQ$$ = d.MINSEQ#)) OR
(d.OLD_NEW$$ = `N` AND (d.SEQ$$ = d.MAXSEQ#)))
(SELECT rowid, a, b, c (Expression 10.1)
FROM t
WHERE rowid NOT IN
(SELECT /*+ CARDINALITY (c1+c2) */
RID$$
FROM DLT_T$$
WHERE OLD_NEW$$ = `N`)
) UNION ALL
(SELECT /*+ CARDINALITY (c3) */
RID$$, a, b, c
FROM DLT_T$$
WHERE OLDNEW$$ = `O`);
wherein c1 = cardinality of new rows from the MV log;
c2 = cardinality of new rows from the direct load log; and
c3 = cardinality of deleted rows from the MV log.
The cardinality hints are useful in choosing between a NESTED LOOP versus a HASH ANTI JOIN operation when accessing the base tables in Expression 10.1. These also assist the optimizer program in choosing the right plans in the main refresh query. Union All Expansion In the presence of DELETEs to multiple base tables, the pre-update state SQL statement, e.g., Expression 5, involves a UNION ALL command. A UNION operation merges the results from any combination of two or more queries, or SELECT statements, or tables, into a single result set, or table, comprising all of the rows belonging to all of the queries, etc. By default, the UNION operation only returns unique records, but using the ALL predicate returns all records, even duplicates. The optimizer may not be able to generate the efficient refresh plan when there are sub-queries with UNION ALL in the FROM clause of a query. To avoid this situation, the UNION ALL command can be expanded. The FROM clause of the query is typically represented as: t1.times.(t2.A-inverted..DELTA.t2); which can be rewritten as (t1.times.t2).orgate.(t1.times..DELTA.t2). Hence, the optimizer program can optimize each branch of the UNION ALL separately. The first branch referencing tables t1 and t2 would have access to the statistics typically already computed for both tables and can utilize any existing indices on the tables. Without this UNION ALL expansion, any index on t2 could not have been used, as it occurs as a UNION (t2.orgate..DELTA.t2). In addition, after the expansion, the branch referencing the small table .DELTA.t2 can benefit from the cardinality hint, described above, passed to the SQL query, and the cardinality hint helps the optimizer program determine the build and probe phases of the Hash join for t1.times.t2. Reducing Recomputation of the Delta MV As described above, once the delta and pre-update states are determined for a base table, they are used to compute the change in the materialized view (delta MV) as expressed in Expression 3. For each component of delta MV resulting from each base table, a refresh pass is performed, through which the materialized view is updated to reflect changes from that component. Each pass can be described as an "apply" operation. There are as many apply operations as the number of table updated. If an INSERT operation performed on a base table contributes to an existing group in the materialized view, the corresponding "apply" operation on the materialized view involves updating the values of aggregates for that group. If an INSERT operation performed on a base table creates a new group in the materialized view, a row must be inserted into the materialized view. Similarly if a DELETE operation is performed on a base table, the corresponding group in the materialized view must be updated. When all rows that contribute to a group are deleted from the base table, the row must be deleted from the materialized view. As previously described, an UPDATE performed on a base table can be treated as a combination of a DELETE of the old value followed by an INSERT of the new value, thus, UPDATES are treated as combinations of INSERTs and DELETEs. Therefore, the application of the delta MV to the materialized view potentially consists of the following three phases. INSERT_PHASE: This phase inserts new rows into the materialized view for any new groups generated by the INSERTs (or UPDATES) to the base tables. UPDATE_PHASE: This phase updates the values of the aggregates to include the value of the aggregates in the delta MV, which includes UPDATES, INSERTS, and DELETES on the base tables. The actual update expression depends on the type of aggregate function. DELETE_PHASE: This phase deletes groups from the materialized view if all rows from the base tables contributing to the group are deleted. A MERGE table operation provides the functionality to insert rows into a table if they were not already present and update them if they were. The MERGE operation can be used to combine two phases above, i.e., the INSERT phase and the UPDATE phase. Use of the MERGE operation reduces the intermediate computations significantly. Hence, whenever both INSERT and UPDATE phases are encountered, they are replaced by a MERGE phase. Ordering Base Tables for Application of Changes to the Materialized View As indicated by Expression 3, to refresh the materialized view, the base tables can each be applied to the materialized view in different orders. The order of application of the base tables to the materialized view can significantly affect the performance of a refresh. For example, considering a materialized view on tables T.sub.1 through T.sub.n, if the changes to these tables are applied to the materialized view in the same order, for table T.sub.i the pre-update state would be computed (i-1) times (while applying changes on T.sub.1 through T.sub.i-1). Therefore, if computing the pre-update state of a table is computationally complex or expensive, the changes for that table should be applied to the materialized view before the changes from the other tables. The general degree of complexity of computing pre-update table states, in decreasing order, is: tables on which conventional operations and direct load inserts have been performed; tables on which a mix of conventional INSERTs and DELETEs, including UPDATES, have been performed (but no direct load INSERTS); and tables on which only INSERTs or only DELETEs have been performed. Base tables with no changes are not considered when refreshing a materialized view. Furthermore, base tables on which other base tables contributing to the materialized view have foreign key constraints and have only INSERTs are also not considered for refresh because these INSERTs would be considered when the base tables which have foreign key constraints are considered for applying refresh. In one embodiment, the base tables are ordered based on the complexity of computing the pre-update state, according to the degree of complexity indicated above. In addition, it is faster to compute the pre-update state for a base table with a smaller cardinality. Thus, considering two base tables with similar complexity of operations, the larger table is considered first when applying refresh. Hardware Overview FIG. 3 is a block diagram that illustrates a computer system 300 upon which an embodiment of the invention may be implemented. Computer system 300 includes a bus 302 or other communication mechanism for communicating information, and a processor 304 rU coupled with bus 302 for processing information. Computer system 300 also includes a main memory 306, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 302 for storing information and instructions to be executed by processor 304. Main memory 306 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 304. Computer system 300 further includes a read only memory (ROM) 308 or other static storage device coupled to bus 302 for storing static information and instructions for processor 304. A storage device 310, such as a magnetic disk, optical disk, or magneto-optical disk, is provided and coupled to bus 302 for storing information and instructions. Computer system 300 may be coupled via bus 302 to a display 312, such as a cathode ray tube (CRT) or a liquid crystal display (LCD), for displaying information to a computer user. An input device 314, including alphanumeric and other keys, is coupled to bus 302 for communicating information and command selections to processor 304. Another type of user input device is cursor control 316, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 304 and for controlling cursor movement on display 312. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. The invention is related to the use of computer system 300 for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 300 in response to processor 304 executing one or more sequences of one or more instructions contained in main memory 306. Such instructions may be read into main memory 306 from another computer-readable medium, such as storage device 310. Execution of the sequences of instructions contained in main memory 306 causes processor 304 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to processor 304 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical, magnetic, or magneto-optical disks, such as storage device 310. Volatile media includes dynamic memory, such as main memory 306. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 302. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 304 for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 300 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 302. Bus 302 carries the data to main memory 306, from which processor 304 retrieves and executes the instructions. The instructions received by main memory 306 may optionally be stored on storage device 310 either before or after execution by processor 304. Computer system 300 also includes a communication interface 318 coupled to bus 302. Communication interface 318 provides a two-way data communication coupling to a network link 320 that is connected to a local network 322. For example, communication interface 318 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 318 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 318 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. Network link 320 typically provides data communication through one or more networks to other data devices. For example, network link 320 may provide a connection through local network 322 to a host computer 324 or to data equipment operated by an Internet Service Provider (ISP) 326. ISP 326 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the "Internet" 328. Local network 322 and Internet 328 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 320 and through communication interface 318, which carry the digital data to and from computer system 300, are exemplary forms of carrier waves transporting the information. Computer system 300 can send messages and receive data, including program code, through the network(s), network link 320 and communication interface 318. As such, a computer such as computer system 300 might transmit a database query or executable code through Internet 328, ISP 326, local network 322 and communication interface 318. The received query or code may be executed by processor 304 as it is received, and/or stored in storage device 310, or other non-volatile storage for later execution. In this manner, computer system 300 may obtain application code in the form of a carrier wave. Extensions and Alternatives Alternative embodiments of the invention are described throughout the foregoing description, and in locations that best facilitate understanding the context of the embodiments. Furthermore, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. In addition, certain process steps are set forth in a particular order, and alphabetic an alphanumeric labels may be used to identify certain steps. Unless specifically stated in the description, embodiments of the invention are not necessarily limited to any particular order of carrying out such steps. In particular, the labels are used merely for convenient identification of steps, and are not intended to specify or require a particular order of carrying out such steps.
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