Oracle手工增加排序区避免SQL使用临时表空间排序产生物理IO
背景
最近公司有个项目需要用到ClickHouse，表是使用列式存储形式的。
对于基本静态的数据或者DML速度要求不高，同时涉及大批量数据集统计的操作相比Oracle来说运行速度大大大幅度提升。
由于Oracle 12C以后开始推出In-Memory特性，可以设置INMEMORY_SIZE 为非0值可启用IM column store特性。
这些都是题外话，好不容易将40多G的表加载到4G的IM区域中，再次执行同样的SQL发现速度还是没有ClickHouse快（虽然比原来快了不少）。
不过对于单纯查询某个列倒是快了几个数量级以上，有兴趣可以点开看看。
10:43:34 ZKM@ODB/pdb(9)> select count(url) from XX_XXXX_XXXXXXX_INFO; COUNT(URL) ---------- 21416307 Elapsed: 00:00:01.86 10:43:39 ZKM@ODB/pdb(9)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last')); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- SQL_ID 481tqtz6srd4n, child number 0 ------------------------------------- select count(url) from XX_XXXX_XXXXXXX_INFO Plan hash value: 2005175113 --------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | --------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 1 |00:00:01.85 | 3 | | 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:01.85 | 3 | | 2 | PARTITION RANGE ALL | | 1 | 21M| 80 |00:00:01.85 | 3 | | 3 | TABLE ACCESS INMEMORY FULL| XX_XXXX_XXXXXXX_INFO | 2 | 21M| 80 |00:00:01.85 | 3 | --------------------------------------------------------------------------------------------------------------- 15 rows selected. Elapsed: 00:00:00.11
消耗才3个逻辑读。
SQL在启用In-Memory特性后的消耗情况如下：
10:49:35 ZKM@ODB/pdb(9)> SELECT 10:49:52 2 COUNT ( 10:49:52 3 DISTINCT CASE 10:49:52 4 WHEN INSTR (URL, '?', 1) > 0 THEN 10:49:52 5 SUBSTR (URL, 0, INSTR(URL, '?', 1) - 1) 10:49:52 6 ELSE 10:49:52 7 URL 10:49:52 8 END 10:49:52 9 ) AS totalPv, 10:49:52 10 COUNT (DISTINCT IP) AS totalUv, 10:49:52 11 COUNT (DISTINCT IP) AS totalIp, 10:49:52 12 TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') AS staticDate 10:49:52 13 FROM 10:49:52 14 XX_XXXX_XXXXXXX_INFO 10:49:52 15 GROUP BY 10:49:52 16 TO_CHAR (CREATE_TIME, 'yyyy-MM-dd'); TOTALPV TOTALUV TOTALIP STATICDATE ---------- ---------- ---------- ------------------------------ 18358 6331 6331 2021-10-15 27798 8426 8426 2021-10-16 26137 9312 9312 2021-10-17 1121 859 859 2021-10-18 40322 16536 16536 2021-10-19 71759 20636 20636 2021-10-20 2085 1489 1489 2021-10-21 76595 23044 23044 2021-10-22 54176 22271 22271 2021-10-23 1501 1358 1358 2021-10-24 63229 19462 19462 2021-10-25 45502 17085 17085 2021-10-26 1245 869 869 2021-10-27 13 rows selected. Elapsed: 00:01:57.69 10:51:50 ZKM@ODB/pdb(9)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- SQL_ID 5cupa20fpf2s2, child number 6 ------------------------------------- SELECT COUNT ( DISTINCT CASE WHEN INSTR (URL, '?', 1) > 0 THEN SUBSTR (URL, 0, INSTR(URL, '?', 1) - 1) ELSE URL END ) AS totalPv, COUNT (DISTINCT IP) AS totalUv, COUNT (DISTINCT IP) AS totalIp, TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') AS staticDate FROM XX_XXXX_XXXXXXX_INFO GROUP BY TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') Plan hash value: 586993993 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 13 |00:01:57.67 | 9 | 13211 | 13211 | | | | | | 1 | SORT GROUP BY | | 1 | 1 | 13 |00:01:57.67 | 9 | 13211 | 13211 | 633K| 633K| 97M (1)| 104M| | 2 | PARTITION RANGE ALL | | 1 | 21M| 21M|00:00:08.25 | 3 | 0 | 0 | | | | | | 3 | TABLE ACCESS INMEMORY FULL| XX_XXXX_XXXXXXX_INFO | 2 | 21M| 21M|00:00:05.81 | 3 | 0 | 0 | | | | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- 20 rows selected. Elapsed: 00:00:00.07
可以看出，时间消耗主要在SORT GROUP BY，即ID=1这一步。
由于该步骤产生了104M的临时数据，基本为物理IO（Writes：13211 * 8 / 1024 = 103.2M）。
想要消除物理IO的影响在看看这条SQL的执行速度。
那么如何消除呢？
排序的时候用到TEMP表空间，无非就是排序区不足。其他中间遇到的乱七八糟的问题和研究就不说了，直接上结论。
设置如下：
ALTER SESSION SET workarea_size_policy = MANUAL;
ALTER SESSION SET sort_area_size = 125829120; （单位bytes，大于前边的104M即可）
再次执行SQL，消耗如下：
11:11:20 ZKM@ODB/pdb(9)> ALTER SESSION SET workarea_size_policy = MANUAL; Session altered. Elapsed: 00:00:00.01 11:11:20 ZKM@ODB/pdb(9)> ALTER SESSION SET sort_area_size = 125829120; Session altered. Elapsed: 00:00:00.00 11:11:25 ZKM@ODB/pdb(9)> SELECT 11:11:35 2 COUNT ( 11:11:35 3 DISTINCT CASE 11:11:35 4 WHEN INSTR (URL, '?', 1) > 0 THEN 11:11:35 5 SUBSTR (URL, 0, INSTR(URL, '?', 1) - 1) 11:11:35 6 ELSE 11:11:35 7 URL 11:11:35 8 END 11:11:35 9 ) AS totalPv, 11:11:35 10 COUNT (DISTINCT IP) AS totalUv, 11:11:35 11 COUNT (DISTINCT IP) AS totalIp, 11:11:35 12 TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') AS staticDate 11:11:35 13 FROM 11:11:35 14 XX_XXXX_XXXXXXX_INFO 11:11:35 15 GROUP BY 11:11:35 16 TO_CHAR (CREATE_TIME, 'yyyy-MM-dd'); TOTALPV TOTALUV TOTALIP STATICDATE ---------- ---------- ---------- ------------------------------ 18358 6331 6331 2021-10-15 27798 8426 8426 2021-10-16 26137 9312 9312 2021-10-17 1121 859 859 2021-10-18 40322 16536 16536 2021-10-19 71759 20636 20636 2021-10-20 2085 1489 1489 2021-10-21 76595 23044 23044 2021-10-22 54176 22271 22271 2021-10-23 1501 1358 1358 2021-10-24 63229 19462 19462 2021-10-25 45502 17085 17085 2021-10-26 1245 869 869 2021-10-27 13 rows selected. Elapsed: 00:01:57.16 11:13:33 ZKM@ODB/pdb(9)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last')); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- SQL_ID 5cupa20fpf2s2, child number 9 ------------------------------------- SELECT COUNT ( DISTINCT CASE WHEN INSTR (URL, '?', 1) > 0 THEN SUBSTR (URL, 0, INSTR(URL, '?', 1) - 1) ELSE URL END ) AS totalPv, COUNT (DISTINCT IP) AS totalUv, COUNT (DISTINCT IP) AS totalIp, TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') AS staticDate FROM XX_XXXX_XXXXXXX_INFO GROUP BY TO_CHAR (CREATE_TIME, 'yyyy-MM-dd') Plan hash value: 586993993 ------------------------------------------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ------------------------------------------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | | 13 |00:01:57.13 | 3 | | | | | 1 | SORT GROUP BY | | 1 | 1 | 13 |00:01:57.13 | 3 | 115M| 18M| 102M (0)| | 2 | PARTITION RANGE ALL | | 1 | 21M| 21M|00:00:08.06 | 3 | | | | | 3 | TABLE ACCESS INMEMORY FULL| XX_XXXX_XXXXXXX_INFO | 2 | 21M| 21M|00:00:05.75 | 3 | | | | ------------------------------------------------------------------------------------------------------------------------------------------ 20 rows selected. Elapsed: 00:00:00.12
已经消除物理IO。
（不过还是慢。。。）