标签
PostgreSQL , 多列条件 , 多索引 , 单列索引 , 复合索引 , 联合索引 , 优化器 , 评估 , 行评估 , 成本
背景
当一个SQL中涉及多个条件，并且多个条件有多种索引可选时，数据库优化器是如何选择使用哪个索引的？
例如
有一张表，有2个字段，单列一个索引，双列一个复合索引.
建表。
postgres=# create table tbl(id int, gid int);
CREATE TABLE
插入1000万记录，其中ID唯一，GID只有10个值。
postgres=# insert into tbl select generate_series(1,10000000), random()*9 ;
INSERT 0 10000000
创建两个索引。
postgres=# create index idx1 on tbl(id);
CREATE INDEX
postgres=# create index idx2 on tbl(gid,id);
CREATE INDEX
下面三条SQL，会如何选择使用哪个索引呢？
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=123;
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=1;
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10);
问题思考
人为选择
这三条QUERY，实际上有三重含义：
1、gid=123的行根本不存在。
如果让你来选索引，你肯定会选复合索引，马上就能定位到数据不存在扫描最少的BLOCK。
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=123;
2、gid=1存在，同时id里面的条件也存在。
如果让你来选索引，应该也是选择复合索引，因为精确定位到了所有的行。
当然如果id in里面很多记录不存在，那么你可能就会选择id单列索引，因为这个索引本身更小，可能扫描更少的BLOCK。
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=1;
3、只有id的条件。
此时，肯定选单列索引了。
select * from tbl where id in (1,2,3,4,5,6,7,8,9,10);
实际情况如何呢？
1、数据库执行计划与预期一致
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=123;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Index Only Scan using idx2 on public.tbl (cost=0.43..2.46 rows=1 width=8) (actual time=0.037..0.037 rows=0 loops=1)
Output: id, gid
Index Cond: (tbl.gid = 123)
Filter: (tbl.id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[]))
Heap Fetches: 0
Buffers: shared hit=3
Planning time: 0.829 ms
Execution time: 0.086 ms
(8 rows)
2、与预期一致
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Index Only Scan using idx2 on public.tbl (cost=0.43..15.46 rows=1 width=8) (actual time=0.026..0.037 rows=2 loops=1)
Output: id, gid
Index Cond: ((tbl.gid = 1) AND (tbl.id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[])))
Heap Fetches: 2
Buffers: shared hit=31
Planning time: 0.121 ms
Execution time: 0.058 ms
(7 rows)
3、与预期一致
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from tbl where id in (1,2,3,4,5,6,7,8,9,10);
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Index Scan using idx1 on public.tbl (cost=0.43..15.52 rows=10 width=8) (actual time=0.021..0.035 rows=10 loops=1)
Output: id, gid
Index Cond: (tbl.id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[]))
Buffers: shared hit=31
Planning time: 0.104 ms
Execution time: 0.055 ms
(6 rows)
问题升华
数据库生成执行计划靠的是统计信息，如果统计信息不准确，那么执行计划必然不准确。
例如我们人为关闭TBL的自动统计信息收集，然后写入一批新的数据。
postgres=# alter table tbl set (autovacuum_enabled =off);
ALTER TABLE
postgres=# insert into tbl select generate_series(1,10000000), 100;
INSERT 0 10000000
这个数据的特点是GID=100，在原有的统计信息中,gid=100的行是不存在的，所以下面的SQL优化器显然做出了错误的决定。
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=100;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
Index Only Scan using idx2 on public.tbl (cost=0.44..2.46 rows=1 width=8) (actual time=0.030..2051.851 rows=10 loops=1)
Output: id, gid
Index Cond: (tbl.gid = 100)
Filter: (tbl.id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[]))
Rows Removed by Filter: 9999990
Heap Fetches: 10000000
Buffers: shared hit=71574
Planning time: 0.130 ms
Execution time: 2051.900 ms
(9 rows)
更新统计信息后，执行计划就准确了。
postgres=# analyze tbl;
ANALYZE
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from tbl where id in (1,2,3,4,5,6,7,8,9,10) and gid=100;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Index Only Scan using idx2 on public.tbl (cost=0.44..20.57 rows=10 width=8) (actual time=0.027..0.043 rows=10 loops=1)
Output: id, gid
Index Cond: ((tbl.gid = 100) AND (tbl.id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[])))
Heap Fetches: 10
Buffers: shared hit=31
Planning time: 0.212 ms
Execution time: 0.067 ms
(7 rows)
如何自动收集统计信息
开启autovacuum , track_counts即可。
有几个微调参数，决定了什么时候扫描是否需要收集统计信息，以及当前表的变化量。
track_counts = on
#------------------------------------------------------------------------------
# AUTOVACUUM PARAMETERS
#------------------------------------------------------------------------------
autovacuum = on # Enable autovacuum subprocess? 'on'
# requires track_counts to also be on.
#log_autovacuum_min_duration = -1 # -1 disables, 0 logs all actions and
# their durations, > 0 logs only
# actions running at least this number
# of milliseconds.
#autovacuum_max_workers = 3 # max number of autovacuum subprocesses
# (change requires restart)
autovacuum_naptime = 3s # time between autovacuum runs
#autovacuum_vacuum_threshold = 50 # min number of row updates before
# vacuum
#autovacuum_analyze_threshold = 50 # min number of row updates before
# analyze
#autovacuum_vacuum_scale_factor = 0.2 # fraction of table size before vacuum
#autovacuum_analyze_scale_factor = 0.1 # fraction of table size before analyze
#autovacuum_freeze_max_age = 200000000 # maximum XID age before forced vacuum
# (change requires restart)
#autovacuum_multixact_freeze_max_age = 400000000 # maximum multixact age
# before forced vacuum
# (change requires restart)
autovacuum_vacuum_cost_delay = 0ms # default vacuum cost delay for
# autovacuum, in milliseconds;
# -1 means use vacuum_cost_delay
#autovacuum_vacuum_cost_limit = -1 # default vacuum cost limit for
# autovacuum, -1 means use
# vacuum_cost_limit
PostgreSQL优化器是支持CBO与遗传算法
评估每个条件过滤多少行
《PostgreSQL pg_stats used to estimate top N freps values and explain rows》
统计信息解读
《PostgreSQL pg_stat_ pg_statio_ 统计信息(scan,read,fetch,hit)源码解读》
《PostgreSQL 统计信息pg_statistic格式及导入导出dump_stat - 兼容Oracle》
升华-多列统计信息
其他因统计信息不准导致的性能问题
《Greenplum 统计信息收集参数 - 暨统计信息不准引入的broadcast motion一例》
其他参考文献
《PostgreSQL bitmapAnd, bitmapOr, bitmap index scan, bitmap heap scan》
《Greenplum 统计信息收集参数 - 暨统计信息不准引入的broadcast motion一例》
《PostgreSQL pg_stat_ pg_statio_ 统计信息(scan,read,fetch,hit)源码解读》
《PostgreSQL 统计信息pg_statistic格式及导入导出dump_stat - 兼容Oracle》
《PostgreSQL pg_stats used to estimate top N freps values and explain rows》
