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postgresql 优化之--不会使用索引，
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--原始表结构如下：
postgres=# \d+ postgres_s1_log
Table "public.postgres_s1_log"
Column | Type | Modifiers | Storage | Stats target | Description
------------------------+--------------------------------+-----------+----------+--------------+-------------
log_time | timestamp(3) without time zone | | plain | |
user_name | text | | extended | |
database_name | text | | extended | |
process_id | integer | | plain | |
connection_from | text | | extended | |
session_id | text | not null | extended | |
session_line_num | bigint | not null | plain | |
command_tag | text | | extended | |
session_start_time | timestamp without time zone | | plain | |
virtual_transaction_id | text | | extended | |
transaction_id | bigint | | plain | |
error_severity | text | | extended | |
sql_state_code | text | | extended | |
message | text | | extended | |
detail | text | | extended | |
hint | text | | extended | |
internal_query | text | | extended | |
internal_query_pos | integer | | plain | |
context | text | | extended | |
query | text | | extended | |
query_pos | integer | | plain | |
location | text | | extended | |
application_name | text | | extended | |
Indexes:
"pk_postgres_s1_log" PRIMARY KEY,btree (session_id,session_line_num)
"idx_error_severity" btree (error_severity)
Has OIDs: no
--原始sql
postgres=# explain
postgres-# select t1.database_name 数据库名,t1.user_name 用户名,cast(substring(t1.message from position(':' in t1.message)+2 for position('ms' in t1.message)-position(':' in t1.message)-2) as float) 执行时间,t2.message 语句,t2.detail 参数,t1.log_time 执行时间,left(t1.connection_from,position(':' in t1.connection_from)-1) 来源IP
postgres-# from postgres_s1_log t1,postgres_s1_log t2 where t1.message like 'duration:%' and t1.user_name<>'postgres' and t1.database_name='dev' and cast(substring(t1.message from position(':' in t1.message)+2 for position('ms' in t1.message)-position(':' in t1.message)-2) as float)>5000
postgres-# and t1.session_id=t2.session_id and t1.session_line_num=t2.session_line_num+1 order by 3 desc limit 500;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------
Limit (cost=820519.37..820520.62 rows=500 width=280)
-> Sort (cost=820519.37..820640.72 rows=48538 width=280)
Sort Key: (("substring"(t1.message,("position"(t1.message,':'::text) + 2),(("position"(t1.message,'ms'::text) - "position"(t1.message,':'::text)) - 2)))::double precision)
-> Merge Join (cost=797111.93..818100.77 rows=48538 width=280)
Merge Cond: ((t2.session_id = t1.session_id) AND (((t2.session_line_num + 1)) = t1.session_line_num))
-> Sort (cost=609126.84..612760.01 rows=1453271 width=188)
Sort Key: t2.session_id,((t2.session_line_num + 1))
-> Seq Scan on postgres_s1_log t2 (cost=0.00..58028.71 rows=1453271 width=188)
-> Materialize (cost=187985.10..189790.94 rows=361169 width=136)
-> Sort (cost=187985.10..188888.02 rows=361169 width=136)
Sort Key: t1.session_id,t1.session_line_num
-> Seq Scan on postgres_s1_log t1 (cost=0.00..105260.02 rows=361169 width=136)
Filter: ((message ~~ 'duration:%'::text) AND (user_name <> 'postgres'::text) AND (database_name = 'dev'::text) AND (("substring"(message,("position"(message,(("position"(message,'ms'::text) - "position"(message,':'::
text)) - 2)))::double precision > 5000::double precision))
--阅读sql可知，该sql是一个自边接，对t1执行主要的过滤，对t2而言依据t1的结果执行过滤取值，且有组合索引 btree (session_id,session_line_num) ，但由于t1.session_line_num=t2.session_line_num+1 至使t2最多只能使用一部分索引
--修改条件 t1.session_line_num=t2.session_line_num+1 为t1.session_line_num -1 =t2.session_line_num，发现t2还是没有使用索引，
--但由于t1与t2进行join时并没有全部使用过滤条件，注意 Seq Scan on postgres_s1_log t1 (cost=0.00..105260.02 rows=361169 width=136)，可发现t1与t2 join，时，pg估计有36万行数据估执行hash join也是正常的，但此时执行计划已发生变化，
postgres=# explain
postgres-# select t1.database_name 数据库名,postgres_s1_log t2 where t1.message like 'duration:%' and t1.user_name<>'postgres' and t1.database_name='dev' and cast(substring(t1.message from position(':' in t1.message)+2 for position('ms' in t1.message)-position(':' in t1.message)-2) as float)>5000
postgres-# and t1.session_id=t2.session_id and t1.session_line_num -1 =t2.session_line_num order by 3 desc limit 500;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------
Limit (cost=807502.65..807503.90 rows=500 width=280)
-> Sort (cost=807502.65..807623.99 rows=48538 width=280)
Sort Key: (("substring"(t1.message,':'::text)) - 2)))::double precision)
-> Hash Join (cost=118146.77..805084.05 rows=48538 width=280)
Hash Cond: ((t1.session_id = t2.session_id) AND ((t1.session_line_num - 1) = t2.session_line_num))
-> Seq Scan on postgres_s1_log t1 (cost=0.00..105260.02 rows=361169 width=136)
Filter: ((message ~~ 'duration:%'::text) AND (user_name <> 'postgres'::text) AND (database_name = 'dev'::text) AND (("substring"(message,':'::text)) - 2))
)::double precision > 5000::double precision))
-> Hash (cost=58028.71..58028.71 rows=1453271 width=188)
-> Seq Scan on postgres_s1_log t2 (cost=0.00..58028.71 rows=1453271 width=188)
--既然已知t1,最多只是返回500条数据，则完全可以让t1先执行过滤再与t2进行join，且t2可以使用索引
postgres=# explain
postgres-# with w1 as (
postgres(# select database_name 数据库名,user_name 用户名,cast(substring(message from position(':' in message)+2 for position('ms' in message)-position(':' in message)-2) as float) 执行用时,log_time 执行时间,left(connection_from,position(':' in connection_from)-1) 来源IP,session_id,session_line_num
postgres(# from postgres_s1_log where message like 'duration:%' and user_name<>'postgres' and database_name='dev'
postgres(# and cast(substring(message from position(':' in message)+2 for position('ms' in message)-position(':' in message)-2) as float)>5000 order by 3 desc limit 500)
postgres-# select 数据库名,用户名,执行用时,执行时间,来源IP from w1,postgres_s1_log t2 where w1.session_id=t2.session_id and w1.session_line_num -1 =t2.session_line_num;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------
Nested Loop (cost=134093.43..138292.75 rows=13 width=278)
CTE w1
-> Limit (cost=134091.75..134093.00 rows=500 width=136)
-> Sort (cost=134091.75..134994.67 rows=361169 width=136)
Sort Key: (("substring"(postgres_s1_log.message,("position"(postgres_s1_log.message,(("position"(postgres_s1_log.message,'ms'::text) - "position"(postgres_s1_log.message,':'::text)) - 2)))::double precision)
-> Seq Scan on postgres_s1_log (cost=0.00..116095.09 rows=361169 width=136)
Filter: ((message ~~ 'duration:%'::text) AND (user_name <> 'postgres'::text) AND (database_name = 'dev'::text) AND (("substring"(message,':'::text)) - 2
)))::double precision > 5000::double precision))
-> CTE Scan on w1 (cost=0.00..10.00 rows=500 width=152)
-> Index Scan using pk_postgres_s1_log on postgres_s1_log t2 (cost=0.43..8.37 rows=1 width=188)
Index Cond: ((session_id = w1.session_id) AND (session_line_num = (w1.session_line_num - 1)))
(10 rows)
--能过原始的cost=807503.90到最后优化的cost=138292.75，性能大概提高了6倍，此时可发现系统的消耗主要在t1,后续需要继续对其进行优化
原文链接：https://www.f2er.com/postgresql/194751.html
