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oracle下一条SQL语句的优化过程(比较详细)

程序员文章站 2023-11-12 20:13:22
原来的语句是这样的:   select sum(sl0000) from xstfxps2 where   dhao00 in (   select dhao00 from...
原来的语句是这样的:

  select sum(sl0000) from xstfxps2 where
  dhao00 in (
  select dhao00 from xstfxps1 where trunc(ywrq00)=trunc(sysdate)
  and khdm00='500000003913');
  已用时间: 00: 02: 49.04
  
  execution plan
  ----------------------------------------------------------
  0 select statement optimizer=choose
  1 0 sort (aggregate)
  2 1 nested loops
  3 2 table access (full) of 'xstfxps2'
  4 2 table access (by index rowid) of 'xstfxps1'
  5 4 index (unique scan) of 'xstfxps1_pk' (unique)
  
  statistics
  ----------------------------------------------------------
  0 recursive calls
  0 db block gets
  17355138 consistent gets
  34141 physical reads
  2912 redo size
  198 bytes sent via sql*net to client
  275 bytes received via sql*net from client
  2 sql*net roundtrips to/from client
  0 sorts (memory)
  0 sorts (disk)
  1 rows processed
  我们看到统计信息里面进行了17355138次逻辑读,34141次物理io,这是相当吓人的数字。在执行计划里面我们看到表xstfxps2来了一次全表扫描。
  我们首先看一下这两张表总的数据量:
  sql> select count(*) from xstfxps2;
  
  count(*)
  ----------
  5585018
  我们这里看到xstfxps2这张表有5585018条记录。
  sql> select count(*) from xstfxps1;
  
  count(*)
  ----------
  702121
  两张表的表结构如下所示:
  sql> desc xstfxps1
  name type nullable default comments
  ------ ------------ -------- ------- --------
  dhao00 number(8)
  lhdh00 number(8) y
  fldh00 number(8) y
  fplb00 varchar2(2) y
  ywrq00 date y
  ywry00 varchar2(8) y
  shrq00 date y
  xsqrrq date y
  xsqrry varchar2(8) y
  khdm00 varchar2(12)
  xkzh00 varchar2(12)
  ckdm00 varchar2(2) y
  thckdm varchar2(2) y
  xsfsdm varchar2(2) y
  fxrydm varchar2(4) y
  shrydm varchar2(4) y
  shbj00 varchar2(1) 'n'
  fxbj00 varchar2(1) 'n'
  skbj00 varchar2(2) y
  fkdm00 varchar2(2) y
  
  sql> desc xstfxps2
  name type nullable default comments
  ------ ------------ -------- ------- --------
  dhao00 number(8)
  spdm00 varchar2(8)
  djia00 number(7,2) 0
  fxsl00 number y 0
  sl0000 number y 0
  thsl00 number y 0
  je0000 number y 0
  se0000 number y
  fpbbh0 varchar2(11) y
  fphao0 varchar2(10) y
  rbdh00 number(8) y
  
  其中xstfxps1的客户订单的表头,保存订单的客户信息、订货日期等信息。xstfxps2是订单的表体,详细记录了客户订单的商品、价格、数量等信息。
  
  调整的第一步是把子查询提取出来,再看语句的执行计划。通常来说,如果语句能够避免子查询的使用,就尽量不用子查询。因为子查询的开销是相当昂贵的。改写后的语句如下:
  select sum(sl0000)
  from xstfxps2 a,(select dhao00 from xstfxps1 where trunc(ywrq00)=trunc(sysdate)
  and khdm00='500000003913') b
  where a.dhao00=b.dhao00;
  已用时间: 00: 00: 03.05
  execution plan
  ----------------------------------------------------------
  0 select statement optimizer=choose
  1 0 sort (aggregate)
  2 1 table access (by index rowid) of 'xstfxps2'
  3 2 nested loops
  4 3 table access (full) of 'xstfxps1'
  5 3 index (range scan) of 'xstfxps2_pk' (unique)
  statistics
  ----------------------------------------------------------
  0 recursive calls
  0 db block gets
  11974 consistent gets
  225 physical reads
  832 redo size
  211 bytes sent via sql*net to client
  275 bytes received via sql*net from client
  2 sql*net roundtrips to/from client
  0 sorts (memory)
  0 sorts (disk)
  1 rows processed
  
  我们可以看到逻辑io由原来的17355138次下降到11974次,有了数量级的提升。执行时间也有原来将近3分钟下降到现在的3秒多一些。很显然性能有了大幅的提升。不过我们看到执行计划里面表xstfxps1还是有一个全表扫描存在。通常来说我们应该尽量避免全表扫描的存在,尤其对于大表,应该建立合适的索引以避免fts的产生。我们来看这两张表的索引信息:
  
  select index_name,column_name from dba_ind_columns where table_name like 'xstfxps%'
  index_name column_name
  ------------------------------ -----------------------------------
  xstfxps1_pk dhao00
  xstfxps2_pk dhao00
  xstfxps2_pk spdm00
  
  我们看到这两张表除了主键约束外都没有建另外的索引。根据语句的查询情况,我们建立了如下的复合索引:
  create index idx_xstfxps1_khdm00_ywrq00 on xstfxps1(khdm00,ywrq00) tablespace indx;
  
  为了使用索引,我们必须对原来的日期字段的条件进行一些调整。因为有个trunc()函数的存在,语句将不会使用到索引。我们只要明白trunc(ywrq00)=trunc(sysdate)事实上等同于ywrq00大于trunc(sysdate),小于trunc(sysdate+1)减去一秒,我们就有了比较好的办法来处理
  这个条件。最终改写后的语句如下:
  select sum(sl0000)
  from xstfxps2 a, xstfxps1 b
  where a.dhao00=b.dhao00
  and b.khdm00='500000003913'
  and b.ywrq00 between trunc(sysdate)
  and trunc(sysdate)+1-1/(24*60*60);
  execution plan
  ----------------------------------------------------------
  0 select statement optimizer=choose
  1 0 sort (aggregate)
  2 1 table access (by index rowid) of 'xstfxps2'
  3 2 nested loops
  4 3 table access (by index rowid) of 'xstfxps1'
  5 4 index (range scan) of 'idx_xstfxps1_khdm00_ywrq00'
  (non-unique)
  
  6 3 index (range scan) of 'xstfxps2_pk' (unique)
  statistics
  ----------------------------------------------------------
  0 recursive calls
  0 db block gets
  3 consistent gets
  0 physical reads
  0 redo size
  210 bytes sent via sql*net to client
  275 bytes received via sql*net from client
  2 sql*net roundtrips to/from client
  0 sorts (memory)
  0 sorts (disk)
  1 rows processed
  我们这时候看逻辑io已经降为3次,语句的执行计划也符合我们的调整目标,创建的索引产生了比较大的效果。这条语句的调整至此告一段落。