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    PostgreSQL 函数调试、诊断、优化 & auto

    xiaoxiao2026-06-18  9

    PostgreSQL 函数调试 & auto_explain

    作者

    digoal

    日期

    2016-11-21

    标签

    PostgreSQL , pldebugger , 函数调试 , auto_explain

    背景

    PostgreSQL的服务端编程能力比较强，不仅支持像java, R, python这种流行的编程语言，另外还内置了一个与Oracle plsql功能几乎一致的plpgsql语言。

    对于开发人员来说，有了语言还不够的，调试是非常常见的需求。

    所以PostgreSQL的函数语言怎么调试呢，使用pldebugger即可。

    pldebugger的安装

    https://git.postgresql.org/gitweb/?p=pldebugger.git;a=summary

    plpgsql调试方法

    https://www.pgadmin.org/docs/1.22/debugger.html

    除了函数的调试，还有函数内SQL的执行计划如何观测。

    auto_explain

    auto_explain 一个比较给力的模块.

    用于向日志输出超出定义阈值执行时间的SQL的执行计划。更强大的是可以选择是否输出内嵌SQL的执行计划(如函数中的SQL).

    auto_explain的输出更详细的解释可以参考EXPLAIN的解释。或者man EXPLAIN.

    SYNOPSIS EXPLAIN [ ( option [, ...] ) ] statement EXPLAIN [ ANALYZE ] [ VERBOSE ] statement where option can be one of: ANALYZE [ boolean ] VERBOSE [ boolean ] COSTS [ boolean ] BUFFERS [ boolean ] FORMAT { TEXT | XML | JSON | YAML }

    开启方式有两种 :

    1. 通过配置文件postgresql.conf

    需要重启数据库。

    2. 通过LOAD 'auto_explain';

    这个只在当前SESSION生效，不需要重启数据库, 需要超级用户权限。

    其实还可以设置local_preload_libraries，session_preload_libraries参数来启用。

    例子

    1. 修改配置文件:

    shared_preload_libraries = 'auto_explain'

    新增如下配置:

    auto_explain.log_min_duration = 0 # 为了方便查看,这里把时间设置为0,所有SQL都会被auto_explain捕获输出.实际使用的时候适当调大。如 100ms auto_explain.log_analyze = true # 以下可选 auto_explain.log_verbose = true auto_explain.log_buffers = true auto_explain.log_nested_statements = true

    然后重启数据库.重启时会输出一个LOG

    postgres@db5-> LOG: loaded library "$libdir/auto_explain.so"

    下面来执行几条SQL

    postgres@db5-> psql -h 127.0.0.1 digoal digoal psql (9.1beta1) Type "help" for help. digoal=> select * from tbl_user_info limit 2; id | firstname | lastname | corp ----+-----------+----------+---------- 1 | zhou1 | digoal1 | sky-mobi 2 | zhou2 | digoal2 | sky-mobi (2 rows) digoal=> select count(*) from tbl_test; count --------- 1000100 (1 row)

    日志输出

    2011-06-08 15:19:14.390 CST,"digoal","digoal",13789,"127.0.0.1:59549",4def2270.35dd,1,"SELECT",2011-06-08 15:19:12 CST,2/18,0,LOG,00 000,"duration: 0.040 ms plan: Query Text: select * from tbl_user_info limit 2; Limit (cost=0.00..0.04 rows=2 width=31) (actual time=0.020..0.023 rows=2 loops=1) Output: id, firstname, lastname, corp Buffers: shared hit=1 -> Seq Scan on public.tbl_user_info (cost=0.00..183.00 rows=10000 width=31) (actual time=0.014..0.015 rows=2 loops=1) Output: id, firstname, lastname, corp Buffers: shared hit=1",,,,,,,,,"psql" 2011-06-08 15:19:17.902 CST,"digoal","digoal",13789,"127.0.0.1:59549",4def2270.35dd,2,"SELECT",2011-06-08 15:19:12 CST,2/19,0,LOG,00 000,"duration: 1232.064 ms plan: Query Text: select count(*) from tbl_test; Aggregate (cost=16927.25..16927.26 rows=1 width=0) (actual time=1232.044..1232.045 rows=1 loops=1) Output: count(*) Buffers: shared hit=4426 -> Seq Scan on public.tbl_test (cost=0.00..14427.00 rows=1000100 width=0) (actual time=0.015..626.872 rows=1000100 loops=1) Output: id Buffers: shared hit=4426",,,,,,,,,"psql"

    2. 通过LOAD 'auto_explain' ;

    这个只在当前SESSION生效，不需要重启数据库, 需要超级用户权限。

    首先先恢复postgresql.conf的配置，去除前面的配置.然后重启数据库.

    普通用户不允许加载auto_explain模块.(普通用户只允许加载$libdir/plugins目录下的模块,但是auto_explain即使拷贝到这个目录也不行)

    digoal=> load 'auto_explain'; ERROR: access to library "auto_explain" is not allowed digoal=> \c digoal postgres You are now connected to database "digoal" as user "postgres". digoal=# load 'auto_explain'; LOAD digoal=# set auto_explain.log_min_duration=0; SET digoal=# select * from tbl_user_info limit 2; id | firstname | lastname | corp ----+-----------+----------+---------- 1 | zhou1 | digoal1 | sky-mobi 2 | zhou2 | digoal2 | sky-mobi (2 rows)

    查看日志

    2011-06-08 15:25:33.361 CST,"postgres","digoal",13968,"127.0.0.1:15445",4def23d5.3690,1,"SELECT",2011-06-08 15:25:09 CST,2/9,0,LOG,0 0000,"duration: 0.048 ms plan: Query Text: select * from tbl_user_info limit 2; Limit (cost=0.00..0.04 rows=2 width=31) -> Seq Scan on tbl_user_info (cost=0.00..183.00 rows=10000 width=31)",,,,,,,,,"psql"

    连接到digoal用户，这个LOAD的模块消失(后面会有解释).

    digoal=# load 'auto_explain'; LOAD digoal=# set session auto_explain.log_min_duration=0; SET digoal=# \c digoal digoal You are now connected to database "digoal" as user "digoal". digoal=> select * from tbl_user_info limit 3; id | firstname | lastname | corp ----+-----------+----------+---------- 1 | zhou1 | digoal1 | sky-mobi 2 | zhou2 | digoal2 | sky-mobi 3 | zhou3 | digoal3 | sky-mobi (3 rows)

    查看日志，没有auto_explain的输出.即使这里使用了session来定义这个参数的有效范围。

    原因是session重新分配了，因为\c digoal digoal 相当于重新连接，backend也重新生成,如下.

    digoal=> select * from pg_stat_activity; datid | datname | procpid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_s tart | xact_start | query_start | waiting | current_query -------+---------+---------+----------+---------+------------------+-------------+-----------------+-------------+------------------ -------------+-------------------------------+-------------------------------+---------+--------------------------------- 16430 | digoal | 14155 | 16423 | digoal | psql | 127.0.0.1 | | 5959 | 2011-06-08 15:31: 32.794906+08 | 2011-06-08 15:31:40.616346+08 | 2011-06-08 15:31:40.616346+08 | f | select * from pg_stat_activity; (1 row) digoal=> \c digoal postgres You are now connected to database "digoal" as user "postgres". digoal=# select * from pg_stat_activity; datid | datname | procpid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_ start | xact_start | query_start | waiting | current_query -------+---------+---------+----------+----------+------------------+-------------+-----------------+-------------+----------------- --------------+-------------------------------+-------------------------------+---------+--------------------------------- 16430 | digoal | 14161 | 10 | postgres | psql | 127.0.0.1 | | 5961 | 2011-06-08 15:31 :43.937297+08 | 2011-06-08 15:31:45.178005+08 | 2011-06-08 15:31:45.178005+08 | f | select * from pg_stat_activity; (1 row)

    那么如何避免这样的情况发生呢? 答案是使用set role .

    digoal=# load 'auto_explain'; LOAD digoal=# set session auto_explain.log_min_duration=0; SET digoal=# set role digoal; SET digoal=> select * from tbl_user_info limit 3; id | firstname | lastname | corp ----+-----------+----------+---------- 1 | zhou1 | digoal1 | sky-mobi 2 | zhou2 | digoal2 | sky-mobi 3 | zhou3 | digoal3 | sky-mobi (3 rows)

    再看日志,已经有了 :

    2011-06-08 15:29:30.011 CST,"postgres","digoal",14062,"127.0.0.1:5939",4def24bc.36ee,1,"SELECT",2011-06-08 15:29:00 CST,2/29,0,LOG,0 0000,"duration: 0.031 ms plan: Query Text: select * from tbl_user_info limit 3; Limit (cost=0.00..0.05 rows=3 width=31) -> Seq Scan on tbl_user_info (cost=0.00..183.00 rows=10000 width=31)",,,,,,,,,"psql"

    如果你不想看日志这么麻烦，想在client直接显示，也很方便，设置client_min_messages='log'就可以看到auto explain的输出了。

    set client_min_messages='log'; set auto_explain.log_min_duration = 0; set auto_explain.log_analyze = true; set auto_explain.log_verbose = true; set auto_explain.log_buffers = true; set auto_explain.log_nested_statements = true; postgres=# do language plpgsql $$ declare begin perform 1 from pg_class where oid=1; end; $$; LOG: duration: 0.008 ms plan: Query Text: SELECT 1 from pg_class where oid=1 Index Only Scan using pg_class_oid_index on pg_catalog.pg_class (cost=0.27..1.29 rows=1 width=4) (actual time=0.006..0.006 rows=0 loops=1) Output: 1 Index Cond: (pg_class.oid = '1'::oid) Heap Fetches: 0 Buffers: shared hit=2 DO

    参考

    1. man LOAD

    2. man EXPLAIN

    3. http://www.postgresql.org/docs/9.6/static/auto-explain.html

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