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2023-04-09
SQL性能优化策略之联合索引优化方法
案例:一条很简单的SQL语句明明选择了索引扫描,但效率还是很低,SQL语句比较简单,是对单张表进行查询,示例代码如下:
SQL> set autot trace SQL> SELECT REQUISITION_ID PARAM1, '1' PARAM2, /*电子标签*/ '1' PARAM3 2 FROM dbo.LIS_REQUISITION_INFO 3 WHERE PRINT_TIME >= 4 TO_DATE('2019-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS') 5 AND PRINT_TIME < SYSDATE 6 and length(requisition_id) = 12 7 AND (TAT1_STATE = '' OR TAT1_STATE IS NULL) 8 AND ROWNUM < 800; Execution Plan ---------------------------------------------------------- Plan hash value: 1151136383 ------------------------------------------------------------------------------------------ | Id | Operation |Name |Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 799 | 18377 | 160K (1)| 00:32:03 | |* 1 | COUNT STOPKEY | | | | | | |* 2 | FILTER | | | | | | |* 3 | TABLE ACCESS BY INDEX ROWID |LIS_REQUISITION_INFO| 800 | 18400 | 160K (1)| 00:32:03 | |* 4 | INDEX RANGE SCAN |I_PRINT_TIME | | | 3799 (1)| 00:00:46 | ------------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<800) 2 - filter(SYSDATE@!>TO_DATE(' 2019-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss')) 3 - filter("TAT1_STATE" IS NULL AND LENGTH("REQUISITION_ID")=12) 4 - access("PRINT_TIME">=TO_DATE(' 2019-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "PRINT_TIME" 从上述代码的执行计划可以看出,Id=4的dbo.LIS_REQUISITION_INFO表选择的索引是I_PRINT_TIME,PRINT_TIME为时间字段,逻辑读高达1204017,下面我们看下该列的选择性,命令如下: SQL> select /*+ NO_MERGE LEADING(a b) */ b.owner, b.table_name, a.column_name, b.num_rows, a.num_distinct Cardinality, ROUND(A.num_distinct * 100 / B.num_rows, 1) selectivity from dba_tab_col_statistics a, dba_tables b where a.owner = b.owner and a.table_name = b.table_name and a.owner = 'DBO' and a.table_name = 'LIS_REQUISITION_INFO' and a.column_name = 'PRINT_TIME'; OWNER TABLE_NAME COLUMN_NAME NUM_ROWS CARDINALITY SELECTIVITY ------- --------------------- ----------- -------- ----------- ----------- DBO LIS_REQUISITION_INFO PRINT_TIME 6933600 2226944 32.1 LIS_REQUISITION_INFO的数据量为6 933 600条,PRINT_TIME列的不同值为2 226 944个,选择性高达32.1%,PRINT_TIME给定了条件时间范围,目前从执行计划来看, LIS_REQUISITION_INFO表的访问先通过I_PRINT_TIME索引进行范围扫描,符合条件的记录回表之后再过滤,产生了大量的单块读。虽然PRINT_TIME的选择性很高,且符合索引扫描的要求,但因为其给定的条件范围太大,导致该字段并不是一个很好的索引选择。 除了PRINT_TIME,该SQL还有requisition_id、TAT1_STATE和ROWNUM,下面就来看下它们的选择性,命令如下: SQL> select /*+ NO_MERGE LEADING(a b) */ b.owner, b.table_name, a.column_name, b.num_rows, a.num_distinct Cardinality, ROUND(A.num_distinct * 100 / B.num_rows, 1) selectivity from dba_tab_col_statistics a, dba_tables b where a.owner = b.owner and a.table_name = b.table_name and a.owner = 'DBO' and a.table_name = 'LIS_REQUISITION_INFO' and a.column_name in ('PRINT_TIME', 'REQUISITION_ID', 'TAT1_STATE'); OWNER TABLE_NAME COLUMN_NAME NUM_ROWS CARDINALITY SELECTIVITY ------- --------------------- -------------------------- ----------- ----------- DBO LIS_REQUISITION_INFO TAT1_STATE 6933600 2 0 DBO LIS_REQUISITION_INFO REQUISITION_ID 6933600 6933600 100 DBO LIS_REQUISITION_INFO PRINT_TIME 6933600 2226944 32.1 SQL> select count(*), from dbo.LIS_REQUISITION_INFO where length(requisition_id) = 12 COUNT(*) ------- 6968919 SQL> select TAT1_STATE, count(*) from dbo.LIS_REQUISITION_INFO group by TAT1_STATE; TAT1_STAT COUNT(*) ---------- -------- 1242217 1 5355366 2 371401 REQUISITION_ID为主键的选择性很高,但几乎所有的记录值都符合length (requisition_id) = 12,TAT1_STATE的数据分布存在倾斜,条件中的TAT1_STATE = '' OR TAT1_STATE IS NULL属于第一种情况,占总数据量的1/3。该字段为固定取值(TAT1_STATE = '' OR TAT1_STATE IS NULL)。如果 PRINT_TIME和TAT1_STATE组合创建联合索引,那么效果又将如何呢?命令如下: 创建索引之后,SQL性能有了明显的提升,逻辑读从原来的1204017降到8008,执行时间也从原来的32分钟降至27秒。 上述案例介绍了简单的复合索引优化,很多情况下,虽然改写SQL能够更好地解决问题,但我们往往很难让开发商去做出修改,因此索引优化变得尤为重要。当表上存在多个过滤条件时,字段在表中的选择性只能作为参考而不能成为最终依据,在实际工作中,我们应该根据业务特点对多个字段进行组合分析。在很多情况下,单个字段的选择性比较低,多个字段的选择性会成倍增长。
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