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Partition Elimination Behavior
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For the query that starts with an index access, whether or not it can eliminate partitions from consideration all really depends on the predicate in your query. A small example will help demonstrate this. The following code creates a table, PARTITIONED_TABLE, that is range partitioned on a numeric column A such that values less than two will be in partition PART_1 and values less than three will be in partition PART_2: ops$tkyte@ORA11GR2> CREATE TABLE partitioned_table 2 ( a int, 3 b int, 4 data char(20) 5 ) 6 PARTITION BY RANGE (a) 7 ( 8 PARTITION part_1 VALUES LESS THAN(2) tablespace p1,
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CHAPTER 13 PARTITIONING
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9 PARTITION part_2 VALUES LESS THAN(3) tablespace p2 10 ) 11 / Table created. We then create both a local prefixed index, LOCAL_PREFIXED, and a local nonprefixed index, LOCAL_NONPREFIXED. Note that the nonprefixed index does not have A on the leading edge of its definition, which is what makes it a nonprefixed index: ops$tkyte@ORA11GR2> create index local_prefixed on partitioned_table (a,b) local; Index created. ops$tkyte@ORA11GR2> create index local_nonprefixed on partitioned_table (b) local; Index created. Next, we ll insert some data into one partition and gather statistics: ops$tkyte@ORA11GR2> insert into partitioned_table 2 select mod(rownum-1,2)+1, rownum, 'x' 3 from all_objects; 72771 rows created. ops$tkyte@ORA11GR2> begin 2 dbms_stats.gather_table_stats 3 ( user, 4 'PARTITIONED_TABLE', 5 cascade=>TRUE ); 6 end; 7 / PL/SQL procedure successfully completed. We take tablespace P2 offline, which contains the PART_2 partition for both the tables and indexes: ops$tkyte@ORA11GR2> alter tablespace p2 offline; Tablespace altered. Taking tablespace P2 offline will prevent Oracle from accessing those specific index partitions. It will be as if we had suffered media failure, causing them to become unavailable. Now we ll query the table to see what index partitions are needed by different queries. This first query is written to permit the use of the local prefixed index: ops$tkyte@ORA11GR2> select * from partitioned_table where a = 1 and b = 1; A B DATA ---------- ---------- -------------------1 1 x This query succeeded, and we can see why by reviewing the explain plan. We ll use the built-in package DBMS_XPLAN to see what partitions this query accesses. The PSTART (partition start) and PSTOP (partition stop) columns in the output show us exactly what partitions this query needs to have online and available in order to succeed:
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ops$tkyte@ORA11GR2> delete from plan_table; 4 rows deleted. ops$tkyte@ORA11GR2> explain plan for 2 select * from partitioned_table where a = 1 and b = 1; Explained. ops$tkyte@ORA11GR2> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ---------------------------------------------------------------------------------| Operation | Name | Rows | Pstart| Pstop | ---------------------------------------------------------------------------------| SELECT STATEMENT | | 1 | | | | PARTITION RANGE SINGLE | | 1 | 1 | 1 | | TABLE ACCESS BY LOCAL INDEX ROWID| PARTITIONED_TABLE | 1 | 1 | 1 | | INDEX RANGE SCAN | LOCAL_PREFIXED | 1 | 1 | 1 | ---------------------------------------------------------------------------------Predicate Information (identified by operation id): --------------------------------------------------3 - access("A"=1 AND "B"=1)
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Note The DBMS_XPLAN output has been edited to remove information that was not relevant, in order to permit
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So, the query that uses LOCAL_PREFIXED succeeds. The optimizer was able to exclude PART_2 of LOCAL_PREFIXED from consideration because we specified A=1 in the query, and we can see that clearly in the plan PSTART and PSTOP are both equal to 1. Partition elimination kicked in for us. The second query fails, however ops$tkyte@ORA11GR2> select * from partitioned_table where b = 1; ERROR: ORA-00376: file 13 cannot be read at this time ORA-01110: data file 13: '/home/ORA11GR2/.../o1_mf_p2_1dzn8jwp_.dbf' no rows selected and using the same technique, we can see why ops$tkyte@ORA11GR2> delete from plan_table; 4 rows deleted. ops$tkyte@ORA11GR2> explain plan for 2 select * from partitioned_table where b = 1; Explained. ops$tkyte@ORA11GR2> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT
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