今天在生产环境中发现一条sql语句尽管走了主键索引，但是查询还是很慢。sql语句类似下面的形式：SELECT /*+ index (bl1_cyc_payer_pop BL1_CYC_PAYER_POP_PK) */ T_TAX.BA_NO, T_TAX.TOTAL_TAX_AMT, T_TAX.TAX_RELATION, T_TAX_ITEM.TAX_ITEM_SEQ_NO, T_TAX_ITEM.TAX_SEQ_NO, T_TAX_ITEM.TAX_AUTHORITY, T_TAX_ITEM.TAX_TYPE, T_TAX_ITEM.TAX_RATE, T_TAX_ITEM.TAX_AMOUNT, T_TAX_ITEM.TAXABLE_AMOUNT, ....... FROM T_TAX, T_TAX_ITEM, T_DOCUMENT, T_CYC_PAYER_POP --这几张都是大表，少则500万左右，多则1000多万。 WHERE T_TAX.TAX_ITEM_PERIOD_KEY = T_TAX_ITEM.PERIOD_KEY AND T_TAX.CUSTOMER_KEY = T_CYC_PAYER_POP.CUSTOMER_KEY AND T_TAX.BA_NO = T_CYC_PAYER_POP.BA_NO AND T_TAX.CYCLE_SEQ_NO = T_CYC_PAYER_POP.CYCLE_SEQ_NO AND T_TAX.CYCLE_SEQ_RUN = T_CYC_PAYER_POP.CYCLE_SEQ_RUN AND T_TAX.PERIOD_KEY = T_CYC_PAYER_POP.PERIOD_KEY AND T_TAX_ITEM.CUSTOMER_KEY = T_CYC_PAYER_POP.CUSTOMER_KEY AND T_TAX_ITEM.TAX_SEQ_NO = T_TAX.TAX_SEQ_NO AND T_DOCUMENT.PERIOD_KEY = T_CYC_PAYER_POP.PERIOD_KEY AND T_DOCUMENT.CUSTOMER_KEY = T_CYC_PAYER_POP.CUSTOMER_KEY AND T_DOCUMENT.BA_NO = T_CYC_PAYER_POP.BA_NO AND T_DOCUMENT.CYCLE_SEQ_NO = T_CYC_PAYER_POP.CYCLE_SEQ_NO AND T_DOCUMENT.CYCLE_SEQ_RUN = T_CYC_PAYER_POP.CYCLE_SEQ_RUN AND T_DOCUMENT.DOC_PRODUCE_IND IN ('Y', 'E') AND T_CYC_PAYER_POP.CUSTOMER_KEY = 78 AND T_CYC_PAYER_POP.PERIOD_KEY = 55 AND T_CYC_PAYER_POP.QA_GROUP = 3 AND T_CYC_PAYER_POP.CYCLE_SEQ_NO = 2925查看该sql的执行计划发现有严重的io问题，瓶颈就在于使用的primary key对应的index

这个问题很值得深究，完全可以使用如下的方式来验证。我尝试使用pk的Hint，另外不加任何hint，看表查询的时候会不会使用index-->使用hint强制走主键查询Plan hash value: 3105767292---------------------------------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |---------------------------------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 12 | 2501 (1)| 00:00:31 | | || 1 | PARTITION RANGE SINGLE | | 1 | 12 | 2501 (1)| 00:00:31 | 171 | 171 ||* 2 | TABLE ACCESS BY LOCAL INDEX ROWID| T_CYC_PAYER_POP | 1 | 12 | 2501 (1)| 00:00:31 | 171 | 171 ||* 3 | INDEX FULL SCAN | T_CYC_PAYER_POP_PK | 541 | | 2444 (1)| 00:00:30 | 171 | 171 |---------------------------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 2 - filter("T_CYC_PAYER_POP"."QA_GROUP"=3) 3 - access("T_CYC_PAYER_POP"."CYCLE_SEQ_NO"=2925 AND "T_CYC_PAYER_POP"."PERIOD_KEY"=55 AND "T_CYC_PAYER_POP"."CUSTOMER_KEY"=78) filter("T_CYC_PAYER_POP"."CUSTOMER_KEY"=78 AND "T_CYC_PAYER_POP"."CYCLE_SEQ_NO"=2925 AND "T_CYC_PAYER_POP"."PERIOD_KEY"=55)Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 31395 consistent gets 0 physical reads 0 redo size 910 bytes sent via SQL*Net to client 520 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 8 rows processed-->来看看不使用hint之后，发生了什么select T_CYC_PAYER_POP.CUSTOMER_KEY, T_CYC_PAYER_POP.PERIOD_KEY, T_CYC_PAYER_POP.QA_GROUP , T_CYC_PAYER_POP.CYCLE_SEQ_NO from T_CYC_PAYER_POP where T_CYC_PAYER_POP.CUSTOMER_KEY = 78 AND T_CYC_PAYER_POP.PERIOD_KEY = 55 AND T_CYC_PAYER_POP.QA_GROUP = 3 AND T_CYC_PAYER_POP.CYCLE_SEQ_NO = 2925 /Execution Plan----------------------------------------------------------Plan hash value: 23637115----------------------------------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |----------------------------------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 12 | 2 (0)| 00:00:01 | | || 1 | PARTITION RANGE SINGLE | | 1 | 12 | 2 (0)| 00:00:01 | 171 | 171 ||* 2 | TABLE ACCESS BY LOCAL INDEX ROWID| T_CYC_PAYER_POP | 1 | 12 | 2 (0)| 00:00:01 | 171 | 171 ||* 3 | INDEX RANGE SCAN | T_CYC_PAYER_POP_5IX | 535 | | 1 (0)| 00:00:01 | 171 | 171 |----------------------------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 2 - filter("T_CYC_PAYER_POP"."CUSTOMER_KEY"=78 AND "T_CYC_PAYER_POP"."PERIOD_KEY"=55) 3 - access("T_CYC_PAYER_POP"."QA_GROUP"=3 AND "T_CYC_PAYER_POP"."CYCLE_SEQ_NO"=2925)Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 951 consistent gets 644 physical reads 80 redo size 910 bytes sent via SQL*Net to client 520 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 8 rows processed启用了index range scan,而且从执行计划和统计信息来看，明显要比全索引扫描效率高得多。可以看到使用index range scan之后，先查询了索引列的信息，然后无法走索引过滤了其他的条件。根据目前的数据情况，这个效率要比全索引效率还高的多。以下是做了hint的改动之后，统计信息的情况，可以看到明显的改善。对于这个Hint的细节需要和客户做更多的确认，毕竟对于调优不能越调越差，稳定和高效才是关键。Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 1178 consistent gets 756 physical reads 0 redo size 3229 bytes sent via SQL*Net to client 553 bytes received via SQL*Net from client 5 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 58 rows processed