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CHAPTER 13 PARTITIONING
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Oracle makes a distinction between the following two types of local indexes: Local prefixed indexes: These are indexes whereby the partition keys are on the leading edge of the index definition. For example, if a table is range partitioned on a column named LOAD_DATE, a local prefixed index on that table would have LOAD_DATE as the first column in its column list. Local nonprefixed indexes: These indexes do not have the partition key on the leading edge of their column list. The index may or may not contain the partition key columns. Both types of indexes are able take advantage of partition elimination, both can support uniqueness (as long as the nonprefixed index includes the partition key), and so on. The fact is that a query that uses a local prefixed index will always allow for index partition elimination, whereas a query that uses a local nonprefixed index might not. This is why local nonprefixed indexes are said to be slower by some people they do not enforce partition elimination (but they do support it). There is nothing inherently better about a local prefixed index as opposed to a local nonprefixed index when that index is used as the initial path to the table in a query. What I mean by that is that if the query can start with scan an index as the first step, there isn t much difference between a prefixed and a nonprefixed index.
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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$tkyte@ORA10G> CREATE TABLE partitioned_table 2 ( a int, 3 b int, 4 data char(20) 5 ) 6 PARTITION BY RANGE (a) 7 ( 8 PARTITION part_1 VALUES LESS THAN(2) tablespace p1, 9 PARTITION part_2 VALUES LESS THAN(3) tablespace p2 10 ) 11 / Table created. We then create both a local prefixed index, LOCAL_PREFIXED, and a local nonprefixed index, LOCAL_NONPREFIXED. Note that the nonprefixed index does not have A on the leading edge of its definition, which is what makes it a nonprefixed index:
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ops$tkyte@ORA10G> create index local_prefixed on partitioned_table (a,b) local; Index created. ops$tkyte@ORA10G> create index local_nonprefixed on partitioned_table (b) local; Index created. Next, we ll insert some data into one partition and gather statistics: ops$tkyte@ORA10G> insert into partitioned_table 2 select mod(rownum-1,2)+1, rownum, 'x' 3 from all_objects; 48967 rows created. ops$tkyte@ORA10G> begin 2 dbms_stats.gather_table_stats 3 ( user, 4 'PARTITIONED_TABLE', 5 cascade=>TRUE ); 6 end; 7 / PL/SQL procedure successfully completed. We take offline tablespace P2, which contains the PART_2 partition for both the tables and indexes: ops$tkyte@ORA10G> 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$tkyte@ORA10G> select * from partitioned_table where a = 1 and b = 1; A B DATA ---------- ---------- -------------------1 1 x That query succeeded, and we can see why by reviewing the explain plan. We ll use the built-in package DBMS_XPLAN to see what partitions this query accesses. The PSTART (partition start) and PSTOP (partition stop) columns in the output show us exactly what partitions this query needs to have online and available in order to succeed: ops$tkyte@ORA10G> delete from plan_table; 4 rows deleted. ops$tkyte@ORA10G> explain plan for 2 select * from partitioned_table where a = 1 and b = 1; Explained.
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