【DB笔试面试677】在Oracle中，对于一个NUMBER(1)的列，若WHERE条件是大于3和大于等于4，这二者是否等价？
- 2019 年 11 月 5 日
- 筆記
题目部分
在Oracle中，对于一个NUMBER(1)的列，如果查询中的WHERE条件分别是大于3和大于等于4，那么这二者是否等价？
♣
答案部分
首先对于查询结果而言，二者没有任何区别。从这一点上讲无论是指定大于3还是指定大于等于4，二者结果都是一样的。但是，结果集一样并不代表二者等价，主要表现为以下几点：
① 在CHECK约束下，如果表属于非SYS用户，那么大于3会执行全表扫描；而大于等于4在经过CHECK约束的检查后，通过FILTER结束查询，能够更高效地返回结果，不用扫描全表。如果表属于SYS用户，那么这二者的执行计划是相同的。因为，若表属于非SYS用户，则最终经过查询转换后的执行SQL为：“SELECT "T_NUM1_LHR"."ID" "ID" FROM "SYS"."T_NUM1_LHR" "T_NUM1_LHR" WHERE "T_NUM1_LHR"."ID">=4 AND 4>4”，而若表属于SYS用户，则最终经过查询转换后的执行SQL为：“SELECT "T_NUM1_LHR"."ID" "ID" FROM "LHR"."T_NUM1_LHR" "T_NUM1_LHR" WHERE "T_NUM1_LHR"."ID">=4”，所以，在非SYS用户下，最终的执行计划中会有“filter(NULL IS NOT NULL)”的谓词条件。
② 在使用索引的时候，由于Oracle索引结构的特点，两者扫描的节点都是从4开始，在执行计划、逻辑读和执行时间等各方面都不存在性能差异。
③ 在使用物化视图的过程中，大于3会同时扫描物化视图和原表，效率较低；而大于等于4会直接扫描物化视图，效率较高。
由此可见，在返回结果集相同的情况下，使用大于等于代替大于在某些特殊情况下可以带来SQL语句性能上的提升。总结一下，如下图所示：
对于这几种情况分别实验如下：
SYS@orclasm > select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production PL/SQL Release 11.2.0.3.0 - Production CORE 11.2.0.3.0 Production TNS for Linux: Version 11.2.0.3.0 - Production NLSRTL Version 11.2.0.3.0 - Production
（一）在CHECK约束下，二者的执行计划是不一样的。
DROP TABLE T_NUM1_LHR; CREATE TABLE T_NUM1_LHR(ID NUMBER(1)); ALTER TABLE T_NUM1_LHR ADD CHECK(ID <4); SET AUTOT ON SELECT * FROM T_NUM1_LHR WHERE ID>3; SELECT * FROM T_NUM1_LHR WHERE ID>=4; LHR@orclasm > SELECT * FROM T_NUM1_LHR WHERE ID>3; no rows selected Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 2700622406 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T_NUM1_LHR | 1 | 13 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("ID">3) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 330 bytes sent via SQL*Net to client 509 bytes received via SQL*Net from client 1 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 0 rows processed LHR@orclasm > SELECT * FROM T_NUM1_LHR WHERE ID>=4; no rows selected Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 3764107410 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 13 | 0 (0)| | |* 1 | FILTER | | | | | | |* 2 | TABLE ACCESS FULL| T_NUM1_LHR | 1 | 13 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(NULL IS NOT NULL) 2 - filter("ID">=4) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 330 bytes sent via SQL*Net to client 509 bytes received via SQL*Net from client 1 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 0 rows processed
如果表中恰好有上面的CHECK约束，那么可以发现，对于大于3和大于等于4这两个SQL的执行计划是不一致的。对于后者，由于查询的条件违反了CHECK约束，因此Oracle在执行计划前面增加了一个FILTER，使得整个查询不需要在执行，因此这个查询不管表中数据有多少，都会在瞬间结束。
而对于大于3这种情况，虽然根据CHECK的约束和列定义，可以推断出这条查询不会返回任何记录，但是Oracle的优化器并没有聪明到根据列的精度来进行分析，因此这个查询会执行全表扫描。也就是说，虽然这两个查询的最终结果一样，但是执行计划并不相同，而且对于大表而言，这种情况下性能也有较大的差别。
当然这种CHECK约束是特例的情况，一般情况下不会出现。原则上到底是选择大于3还是大于等于4，应该根据具体的业务来决定，而不要尝试利用Oracle的数据精度来设置查询条件。如果以后一旦字段的结构发生了修改，比如这个例子中字段的允许出现小数，那么这两个SQL的WHERE条件就不再等价了。
若表属于SYS用户，则这二者的执行计划是相同的。
下面通过10053事件查看具体原因：
ALTER SESSION SET EVENTS '10053 trace name context forever, level 1'; SELECT * FROM T_NUM1_LHR WHERE ID >= 4; ALTER SESSION SET EVENTS '10053 trace name context off'; SELECT VALUE FROM V$DIAG_INFO WHERE NAME='Default Trace File';
SYS用户：
try to generate transitive predicate from check constraints for query block SEL$1 (#0) finally: "T_NUM1_LHR"."ID">=4 apadrv-start sqlid=4141557682765762850 : call(in-use=1400, alloc=16344), compile(in-use=54632, alloc=55568), execution(in-use=2480, alloc=4032) ******************************************* Peeked values of the binds in SQL statement ******************************************* Final query after transformations:******* UNPARSED QUERY IS ******* SELECT "T_NUM1_LHR"."ID" "ID" FROM "SYS"."T_NUM1_LHR" "T_NUM1_LHR" WHERE "T_NUM1_LHR"."ID">=4 kkoqbc: optimizing query block SEL$1 (#0)
普通用户：
try to generate transitive predicate from check constraints for query block SEL$1 (#0) constraint: "T_NUM1_LHR"."ID"<4 finally: "T_NUM1_LHR"."ID">=4 AND 4>4 FPD: transitive predicates are generated in query block SEL$1 (#0) "T_NUM1_LHR"."ID">=4 AND 4>4 apadrv-start sqlid=11964066854041036881 : call(in-use=1696, alloc=16344), compile(in-use=55176, alloc=58488), execution(in-use=2744, alloc=4032) ******************************************* Peeked values of the binds in SQL statement ******************************************* Final query after transformations:******* UNPARSED QUERY IS ******* SELECT "T_NUM1_LHR"."ID" "ID" FROM "LHR"."T_NUM1_LHR" "T_NUM1_LHR" WHERE "T_NUM1_LHR"."ID">=4 AND 4>4 kkoqbc: optimizing query block SEL$1 (#0)
（二）在有索引的情况下，二者的性能是否有差异
DROP TABLE T_NUM2_LHR; CREATE TABLE T_NUM2_LHR(ID NUMBER,NAME VARCHAR2(30)); CREATE INDEX IND_TNUM2_ID ON T_NUM2_LHR(ID); INSERT INTO T_NUM2_LHR SELECT 3，OBJECT_NAME FROM DBA_OBJECTS; INSERT INTO T_NUM2_LHR SELECT * FROM T_NUM2_LHR; INSERT INTO T_NUM2_LHR SELECT * FROM T_NUM2_LHR; INSERT INTO T_NUM2_LHR SELECT * FROM T_NUM2_LHR; INSERT INTO T_NUM2_LHR SELECT * FROM T_NUM2_LHR; COMMIT; INSERT INTO T_NUM2_LHR VALUES(4,'test'); COMMIT; SET TIMING ON SET AUTOT ON SELECT * FROM T_NUM2_LHR WHERE ID>3; SELECT * FROM T_NUM2_LHR WHERE ID>=4; LHR@orclasm > SELECT * FROM T_NUM2_LHR WHERE ID>3; ID NAME ---------- ------------------------------ 4 test Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 4021107501 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 30 | 1 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T_NUM2_LHR | 1 | 30 | 1 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IND_TNUM2_ID | 1 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("ID">3) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 595 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) 1 rows processed LHR@orclasm > SELECT * FROM T_NUM2_LHR WHERE ID>=4; ID NAME ---------- ------------------------------ 4 test Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 4021107501 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 30 | 1 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T_NUM2_LHR | 1 | 30 | 1 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IND_TNUM2_ID | 1 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("ID">=4) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 595 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) 1 rows processed
可以看到，无论是执行时间，还是逻辑读，两个SQL没有任何的差别。根据Oracle索引结构的特点，无论是大于3还是大于等于4，这二者的查询所扫描的叶节点都是同一个，因此，在这一点上不会存在性能的差别。
（三）在使用物化视图上的差别
如果表上建立了可查询重写的物化视图，那么这两个查询在是否使用物化视图上有所差别。
CREATE TABLE T_NUM3_LHR(ID NUMBER,NUM NUMBER(1)); ALTER TABLE T_NUM3_LHR ADD PRIMARY KEY(ID); INSERT INTO T_NUM3_LHR SELECT ROWNUM,MOD(ROWNUM,4) FROM DBA_OBJECTS; INSERT INTO T_NUM3_LHR SELECT ROWNUM+54916,MOD(ROWNUM,4) FROM T_NUM3_LHR; INSERT INTO T_NUM3_LHR SELECT ROWNUM+109832,MOD(ROWNUM,4) FROM T_NUM3_LHR; INSERT INTO T_NUM3_LHR SELECT ROWNUM+219664,MOD(ROWNUM,4) FROM T_NUM3_LHR; INSERT INTO T_NUM3_LHR SELECT ROWNUM+439328,MOD(ROWNUM,4) FROM T_NUM3_LHR; COMMIT; INSERT INTO T_NUM3_LHR VALUES(1000000,4); COMMIT; SET AUTOT ON SELECT * FROM T_NUM3_LHR WHERE NUM>3; SELECT * FROM T_NUM3_LHR WHERE NUM>=4; LHR@orclasm > SET AUTOT ON LHR@orclasm > SELECT * FROM T_NUM3_LHR WHERE NUM>3; ID NUM ---------- ---------- 1000000 4 Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 621453705 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 12 | 312 | 314 (3)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T_NUM3_LHR | 12 | 312 | 314 (3)| 00:00:04 | -------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("NUM">3) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 1 db block gets 1150 consistent gets 0 physical reads 0 redo size 588 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) 1 rows processed LHR@orclasm > SELECT * FROM T_NUM3_LHR WHERE NUM>=4; ID NUM ---------- ---------- 1000000 4 Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 621453705 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 12 | 312 | 314 (3)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T_NUM3_LHR | 12 | 312 | 314 (3)| 00:00:04 | -------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("NUM">=4) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 1 db block gets 1150 consistent gets 0 physical reads 0 redo size 588 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) 1 rows processed
由于采用的都是全表扫描，二者执行的时间和逻辑读完全一样。
下面建立一个物化视图：
SET AUTOT OFF CREATE MATERIALIZED VIEW LOG ON T_NUM3_LHR WITH(NUM); CREATE MATERIALIZED VIEW MV_T_NUM3_LHR REFRESH FAST ENABLE QUERY REWRITE AS SELECT ID,NUM FROM T_NUM3_LHR WHERE NUM>=4; LHR@orclasm > SET AUTOT OFF LHR@orclasm > CREATE MATERIALIZED VIEW LOG ON T_NUM3_LHR WITH(NUM); Materialized view log created. LHR@orclasm > CREATE MATERIALIZED VIEW MV_T_NUM3_LHR REFRESH FAST ENABLE QUERY REWRITE AS SELECT ID,NUM FROM T_NUM3_LHR WHERE NUM>=4; Materialized view created. LHR@orclasm > show parameter query NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ query_rewrite_enabled string TRUE query_rewrite_integrity string enforced LHR@orclasm > SET AUTOT ON LHR@orclasm > SELECT * FROM T_NUM3_LHR WHERE NUM>3; ID NUM ---------- ---------- 1000000 4 Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- SELECT * FROM T_NUM3_LHR WHERE NUM>=4; Plan hash value: 4012093353 ------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 13 | 338 | 317 (3)| 00:00:04 | | 1 | VIEW | | 13 | 338 | 317 (3)| 00:00:04 | | 2 | UNION-ALL | | | | | | | 3 | MAT_VIEW REWRITE ACCESS FULL| MV_T_NUM3_LHR | 1 | 26 | 3 (0)| 00:00:01 | |* 4 | TABLE ACCESS FULL | T_NUM3_LHR | 12 | 312 | 314 (3)| 00:00:04 | ------------------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 4 - filter("NUM">3 AND "NUM"<4) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 1 db block gets 1153 consistent gets 0 physical reads 0 redo size 588 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) 1 rows processed LHR@orclasm > ID NUM ---------- ---------- 1000000 4 Elapsed: 00:00:00.00 Execution Plan ---------------------------------------------------------- Plan hash value: 4274348025 ---------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 26 | 3 (0)| 00:00:01 | | 1 | MAT_VIEW REWRITE ACCESS FULL| MV_T_NUM3_LHR | 1 | 26 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------- Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 592 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) 1 rows processed
从执行计划可以看到，对于大于等于4的情况，Oracle直接扫描了物化视图了。而对于大于3的情况，Oracle同时扫描了物化视图和原表，显然效率比较低。
这个例子其实和第一个例子很类似。虽然根据字段类型可以判断出大于3和大于等于4是等价的，但是对于CBO来说，并不会将数据类型的因素考虑进去。因此导致两个查询在使用物化视图时执行计划的区别。
本文选自《Oracle程序员面试笔试宝典》，作者：小麦苗
