Oracle index】SQL语句无法走索引的一些情况分析及语句改写思路

因为优化器还不够强大，还有很多限制，或者因为一些逻辑原因，分析认为SQL要走索引比较好，但是事实却无法正确利用索引。这时候，除了给ORACLE需要的统计信息之外，写的SQL必须要能够给优化器足够多的额外有效信息，让优化器能够选择更好的执行计划。

要让给优化器正确使用上需要的索引，要考虑两点：

1).如何避免优化器的限制 2).根据业务数据特点改写SQL语句



说明：这里说的走不了索引，是指走不了正常的RANGE SCAN，非(FAST) FULL INDEX SCAN。

1.谓词使用不等于(<>,!=)，目前的优化器一般认为不等于的选择性不好，会找到很多行，所以用不上索引,就算强制索引，也是扫描全部的索引，走不了RANGE SCAN

如果事实走索引定位较好，那么解决方法：

1）如果不等条件之外的值不多，而且是确认的，可以改为等值或IN查询,比如status状态字段一般值类别很少

2）如果不等条件之外的值很多，可以改为> OR <的形式，当然第2种方法包含了方法1.

例如：

DROP TABLE t; CREATE TABLE t AS SELECT LEVEL ID,rpad('x',10,'x') padding,CAST('JACK' AS VARCHAR2(100)) NAME FROM dual CONNECT BY LEVEL<100000; --构造3行较少的值 INSERT INTO t VALUES(100000,'y','DINGJUN1'); INSERT INTO t VALUES(100001,'y','DINGJUN2'); INSERT INTO t VALUES(100002,'y','DINGJUN3'); COMMIT; ALTER TABLE t MODIFY NAME NOT NULL; --创建索引 CREATE INDEX idx_t ON t(NAME); BEGIN dbms_stats.gather_table_stats(ownname => USER,tabname => 'T',estimate_percent => 100,method_opt => 'for columns name size 10',cascade => TRUE); END; /

<>无法利用索引：

dingjun123@ORADB> SELECT * FROM t 2 WHERE t.name<>'JACK'; 3 rows selected. Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 4 | 88 | 107 (2)| 00:00:02 | |* 1 | TABLE ACCESS FULL| T | 4 | 88 | 107 (2)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("T"."NAME"<>'JACK')

将<>改写为OR连接后，能够正确使用索引，走OR扩展：

dingjun123@ORADB> SELECT * FROM t 2 WHERE t.NAME>'JACK' OR t.NAME <'JACK'; 3 rows selected. Elapsed: 00:00:00.06 Execution Plan ---------------------------------------------------------- Plan hash value: 2945726203 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 4 | 88 | 6 (0)| 00:00:01 | | 1 | CONCATENATION | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 1 | 22 | 3 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_T | 1 | | 2 (0)| 00:00:01 | | 4 | TABLE ACCESS BY INDEX ROWID| T | 3 | 66 | 3 (0)| 00:00:01 | |* 5 | INDEX RANGE SCAN | IDX_T | 4 | | 2 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("T"."NAME">'JACK') 5 - access("T"."NAME"<'JACK') filter(LNNVL("T"."NAME">'JACK'))

如果业务允许，改为写下列语句也是走索引的，不再演示。

SELECT * FROM t

WHERE t.NAME IN ('DINGJUN1','DINGJUN2','DINGJUN3');

NOT IN,NOT EXISTS与<>类似，也是走不了索引的（注意，这里说走不了索引，只针对于不等连接谓词，不包括其他谓词）

演示NOT IN：

DROP TABLE t1; CREATE TABLE t1 AS SELECT 1 ID,'JACK' NAME FROM dual; ALTER TABLE t1 MODIFY NAME NOT NULL; BEGIN dbms_stats.gather_table_stats(ownname => USER,tabname => 'T1',estimate_percent => 100); END; /

NOT IN也不能使用索引，强制使用只能扫描全部索引：

dingjun123@ORADB> SELECT * FROM t 2 WHERE t.NAME NOT IN(SELECT t1.NAME FROM t1); Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 4253491563 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 100K| 2636K| 110 (2)| 00:00:02 | |* 1 | HASH JOIN RIGHT ANTI| | 100K| 2636K| 110 (2)| 00:00:02 | | 2 | TABLE ACCESS FULL | T1 | 1 | 5 | 3 (0)| 00:00:01 | | 3 | TABLE ACCESS FULL | T | 100K| 2148K| 106 (1)| 00:00:02 | ----------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("T"."NAME"="T1"."NAME") dingjun123@ORADB> SELECT/*+index(t idx_t)*/ * FROM t 2 WHERE t.NAME NOT IN(SELECT t1.NAME FROM t1); Elapsed: 00:00:00.03 Execution Plan ---------------------------------------------------------- Plan hash value: 1707532828 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 100K| 2636K| 591 (1)| 00:00:08 | | 1 | MERGE JOIN ANTI | | 100K| 2636K| 591 (1)| 00:00:08 | | 2 | TABLE ACCESS BY INDEX ROWID| T | 100K| 2148K| 587 (1)| 00:00:08 | | 3 | INDEX FULL SCAN | IDX_T | 100K| | 225 (1)| 00:00:03 | |* 4 | SORT UNIQUE | | 1 | 5 | 4 (25)| 00:00:01 | | 5 | TABLE ACCESS FULL | T1 | 1 | 5 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 4 - access("T"."NAME"="T1"."NAME") filter("T"."NAME"="T1"."NAME")

NOT EXISTS与NOT IN类似（注意，NOT IN已经处理过NULL的影响）

SELECT * FROM t

WHERE NOT EXISTS(SELECT 1 FROM t1 WHERE t.NAME=t1.NAME);

2.由于B*Tree索引不存储全为NULL的行，造成无法走索引的情况

解决方法：

1）视业务要求是否可以不统计NULL的

加条件where xxx IS NOT NULL, COUNT(列).....等等过滤NULL的语句

2）复合列索引

3）函数索引

详细可以参考【Oracle index】B*Tree索引之NULL统计技巧 这个帖子里的内容。

3.LIKE前通配查询询，LIKE全通配查询走不了索引

解决方法：

1）是否可以根据业务需求把前通配去掉

dingjun123@ORADB> SELECT * FROM t WHERE t.NAME LIKE '%DINGJUN%'; Elapsed: 00:00:00.04 Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 5000 | 107K| 107 (2)| 00:00:02 | |* 1 | TABLE ACCESS FULL| T | 5000 | 107K| 107 (2)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("T"."NAME" LIKE '%DINGJUN%') --改为后通配，走索引 dingjun123@ORADB> SELECT * FROM t WHERE t.NAME LIKE 'DINGJUN%'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 1594971208 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 66 | 3 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 3 | 66 | 3 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_T | 3 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("T"."NAME" LIKE 'DINGJUN%') filter("T"."NAME" LIKE 'DINGJUN%')

2) 是否和此LIKE一样的前通配或全通配的SQL有很多，此谓词的LIKE变化不大，如果是，考虑建立函数索引，否则对于全通配问题最好办法就是全文索引

dingjun123@ORADB> CREATE INDEX idx1_t ON t (instr(NAME,'DINGJUN')); Index created. Elapsed: 00:00:00.16dingjun123@ORADB> SELECT * FROM t WHERE instr(t.NAME,'DINGJUN')>0; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 2071967826 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 5000 | 102K| 7 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 5000 | 102K| 7 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX1_T | 900 | | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access(INSTR("NAME",'DINGJUN')>0)

3)如果只是前通配，可以使用reverse函数索引（不是翻转键索引）

SELECT * FROM t WHERE t.NAME LIKE '%DINGJUN1';

CREATE INDEX idx2_t ON t(REVERSE(NAME));

语句要改写,使用reverse函数谓词，并且查找值倒查：

dingjun123@ORADB> SELECT * FROM t WHERE REVERSE(t.NAME) LIKE '1NUJGNID%'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 3787301248 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 5000 | 102K| 8 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 5000 | 102K| 8 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX2_T | 900 | | 4 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access(REVERSE("NAME") LIKE '1NUJGNID%') filter(REVERSE("NAME") LIKE '1NUJGNID%')

注意：如果查找的是中文，得注意，不可直接写 REVERSE(t.NAME) LIKE '1NUJGNID%'，因为REVERSE内部会按字节翻转，中文的写法可以用REVERSE转换，比如

SELECT * FROM t WHERE REVERSE(t.name) LIKE REVERSE('数据')||'%';否则查询出的数据不对。

4.对索引列使用了函数，数学运算，其他表达式等

解决方法：去掉对索引列的相关运算，保持索引列纯净。

优化器目前对一些数学运算，还无法做很好的消除动作，所以对于索引列应该尽量保持纯净，否则可能无法用上正确的索引

dingjun123@ORADB> CREATE INDEX idx3_t ON t(ID); Index created. Elapsed: 00:00:00.13 dingjun123@ORADB> ALTER TABLE T MODIFY ID NOT NULL; Table altered. Elapsed: 00:00:00.10 dingjun123@ORADB> SELECT * FROM t WHERE ID+0=1; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1000 | 21000 | 107 (2)| 00:00:02 | |* 1 | TABLE ACCESS FULL| T | 1000 | 21000 | 107 (2)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("ID"+0=1) dingjun123@ORADB> SELECT * FROM t WHERE ID=1; Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 2351669764 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1000 | 21000 | 3 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 1000 | 21000 | 3 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX3_T | 400 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("ID"=1)

举例只是举简单的数学运算，可能运算还和其他列运算，比如where ID+ext_col...

5.ORACLE使用了隐式类型转换,导致索引无法使用

解决方法：

必须避免隐式类型转换，全部要求显式类型转换（非索引列），且避免对索引列进行类型转换（有函数索引除外）

如果类型不一致，不管是否发生自动类型转换，谓词的右值应该显式转换为与索引列保持一致（对于非索引列的运算也应该如此）。

dingjun123@ORADB> DROP TABLE t1; Table dropped. Elapsed: 00:00:00.03 dingjun123@ORADB> CREATE TABLE t1(x VARCHAR2(100)); Table created. Elapsed: 00:00:00.04 dingjun123@ORADB> CREATE INDEX idx_t1 ON t1(x); Index created. Elapsed: 00:00:00.04 dingjun123@ORADB> SELECT * FROM t1 WHERE x = 1; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 3617692013 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 52 | 2 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T1 | 1 | 52 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(TO_NUMBER("X")=1) dingjun123@ORADB> SELECT * FROM t1 WHERE x = '1'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 1369807930 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 52 | 1 (0)| 00:00:01 | |* 1 | INDEX RANGE SCAN| IDX_T1 | 1 | 52 | 1 (0)| 00:00:01 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("X"='1')

发生类型转换，可以通过执行计划中的谓词信息获知。通过分析发现，X因为是VARCHAR2，优先级比数值类型低，遇到数值类型，会TO_NUMBER,所以索引失效，通过传入与索引列类型一致的字符串后，解决。

6.对于其他不走索引的，尽量通过业务的逻辑特点改写语句，可能正确利用索引

例如一个典型的例子：

DROP TABLE t_objects; CREATE TABLE t_objects AS SELECT * FROM dba_objects; CREATE INDEX idx_t_objects ON t_objects(last_ddl_time,created); BEGIN dbms_stats.gather_table_stats(ownname => USER,tabname => 't_objects',estimate_percent => 100,cascade => TRUE); END; /

需求：查找创建时间是2013年的，并且最后ddl时间比创建时间大1天以上。

dingjun123@ORADB> SELECT * FROM 2 t_objects t 3 WHERE t.last_ddl_time-t.created>1 4 AND t.created>=DATE'2013-1-1'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 3629755566 ------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1171 | 110K| 300 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T_OBJECTS | 1171 | 110K| 300 (1)| 00:00:04 | ------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME"-"T"."CREATED">1)

这个索引是组合索引，上面的语句对前导列进行了运行，也不符合走index skip scan的条件，所以，走FULL TABLE SCAN。那么是否可以通过逻辑改写走索引呢，通过第4点得知，要保持索引列清净，将create_date移到右边，语句如下：

dingjun123@ORADB> SELECT * FROM 2 t_objects t 3 WHERE t.last_ddl_time>=(t.created+1) 4 AND t.created>=DATE'2013-1-1'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 3629755566 ------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 403 | 39091 | 301 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T_OBJECTS | 403 | 39091 | 301 (1)| 00:00:04 | ------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME">=INTERNAL_FUNCTION("T" ."CREATED")+1)

通过改写后发现，还是没有走索引，因为ORACLE认为前导列右边的created不固定，无法从指定索引处查找。通过这个分析得知，ORACLE谓词传递有一定限制，create_date+1无法做谓词传递给last_ddl_time,改写：

dingjun123@ORADB> SELECT * FROM 2 t_objects t 3 WHERE t.last_ddl_time>=(t.created) 4 AND t.last_ddl_time>=(t.created+1) 5 AND t.created>=DATE'2013-1-1'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 641904483 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 194 | 11 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T_OBJECTS | 2 | 194 | 11 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_T_OBJECTS | 6 | | 10 (0)| 00:00:01 | --------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("T"."LAST_DDL_TIME">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME" IS NOT NULL) filter("T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME">="T"."CREATED" AND "T"."LAST_DDL_TIME">=INTERNAL_FUNCTION("T"."CREATED")+1)

上面的oracle知道谓词传递给last_ddl_time了，T"."LAST_DDL_TIME">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss')，也可以手动谓词传递，last_ddl_time肯定大于等于DATE'2013-1-2'：

dingjun123@ORADB> SELECT * FROM 2 t_objects t 3 WHERE t.last_ddl_time>=DATE'2013-1-2' 4 AND t.last_ddl_time>=(t.created+1) 5 AND t.created>=DATE'2013-1-1'; Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 641904483 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 13 | 1261 | 11 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T_OBJECTS | 13 | 1261 | 11 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_T_OBJECTS | 13 | | 10 (0)| 00:00:01 | --------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("T"."LAST_DDL_TIME">=TO_DATE(' 2013-01-02 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME" IS NOT NULL) filter("T"."CREATED">=TO_DATE(' 2013-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND "T"."LAST_DDL_TIME">=INTERNAL_FUNCTION("T"."CREATED")+1)

如果查询条件无t.created>=DATE'2013-1-2',如下面语句：

SELECT * FROM t_objects t WHERE t.last_ddl_time-t.created>1; QL> set autotrace traceonly exp QL> SELECT * FROM 2 t_objects t 3 WHERE t.last_ddl_time-t.created>1; 执行计划 --------------------------------------------------------- Plan hash value: 3629755566 ------------------------------------------------------------------------------ Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------ 0 | SELECT STATEMENT | | 11706 | 1108K| 297 (2)| 00:00:04 | * 1 | TABLE ACCESS FULL| T_OBJECTS | 11706 | 1108K| 297 (2)| 00:00:04 | ------------------------------------------------------------------------------ redicate Information (identified by operation id): -------------------------------------------------- 1 - filter("T"."LAST_DDL_TIME"-"T"."CREATED">1) SQL> set autotrace off SQL> SELECT COUNT(*) FROM 2 t_objects t; COUNT(*) ---------- 74059 SQL> SELECT COUNT(*) FROM 2 t_objects t 3 WHERE t.last_ddl_time-t.created>1; COUNT(*) ---------- 216 应该走索引更佳，因为没有其他过滤条件，可以考虑建立函数索引： SQL> CREATE INDEX idx1_t_object ON t_objects(last_ddl_time-created); 索引已创建。 --注意收集直方图，因为分布不均 SQL> exec dbms_stats.gather_table_stats(ownname => USER,tabname => 't_objects',estimate_percent => 100,method_opt => 'for all indexed columns',cascade => TRUE); PL/SQL 过程已成功完成。 SQL> SELECT COUNT(*) FROM 2 t_objects t 3 WHERE t.last_ddl_time-t.created>1; 执行计划 ---------------------------------------------------------- Plan hash value: 3236535878 ----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 9 | 4 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 9 | | | |* 2 | INDEX RANGE SCAN| IDX1_T_OBJECT | 986 | 8874 | 4 (0)| 00:00:01 | ----------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("LAST_DDL_TIME"-"CREATED">1) 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 424 bytes sent via SQL*Net to client 416 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed

当然，对于两个都是范围的查询，这里只能通过一个列来轮询索引，先做access,再做filter。SQL语句的逻辑改写很重要，往往通过逻辑改写就能改变SQL的执行计划，从不好的计划到好的计划，比如semi join,anti join与or,往往走FILTER导致执行计划较差，这时候就需要通过逻辑等价改写，逻辑等价改写往往需要掌握一些集合的知识，比如NOT (A AND B)===NOT A OR NOT B,NOT (A OR B)===NOT A AND NOT B等。