PgSQL技术内幕-Analyze做的那些事-pg_stat_all_tables
pg_stat_all_tables视图中记录有analyze信息，比如何时做的analyze、表元组个数（活元组、死元组）等。重启后发现该视图中表的统计信息重置不见了，发生了什么？
1、pg_stat_all_tables
先理解下该视图，该视图定义如下。从视图定义可以看到相关统计信息都是通过一系列函数获取的：
yzs=# d+ pg_stat_all_tables 视图 "pg_catalog.pg_stat_all_tables" 栏位 | 类型 | 校对规则 | 可空的 | 预设 | 存储 | 描述---------------------+--------------------------+----------+--------+------+-------+------ relid | oid | | | | plain | schemaname | name | | | | plain | relname | name | | | | plain | seq_scan | bigint | | | | plain | seq_tup_read | bigint | | | | plain | idx_scan | bigint | | | | plain | idx_tup_fetch | bigint | | | | plain | n_tup_ins | bigint | | | | plain | n_tup_upd | bigint | | | | plain | n_tup_del | bigint | | | | plain | n_tup_hot_upd | bigint | | | | plain | n_live_tup | bigint | | | | plain | n_dead_tup | bigint | | | | plain | n_mod_since_analyze | bigint | | | | plain | last_vacuum | timestamp with time zone | | | | plain | last_autovacuum | timestamp with time zone | | | | plain | last_analyze | timestamp with time zone | | | | plain | last_autoanalyze | timestamp with time zone | | | | plain | vacuum_count | bigint | | | | plain | autovacuum_count | bigint | | | | plain | analyze_count | bigint | | | | plain | autoanalyze_count | bigint | | | | plain |视图定义: SELECT c.oid AS relid, n.nspname AS schemaname, c.relname, pg_stat_get_numscans(c.oid) AS seq_scan, pg_stat_get_tuples_returned(c.oid) AS seq_tup_read, sum(pg_stat_get_numscans(i.indexrelid))::bigint AS idx_scan, sum(pg_stat_get_tuples_fetched(i.indexrelid))::bigint + pg_stat_get_tuples_fetched(c.oid) AS idx_tup_fetch, pg_stat_get_tuples_inserted(c.oid) AS n_tup_ins, pg_stat_get_tuples_updated(c.oid) AS n_tup_upd, pg_stat_get_tuples_deleted(c.oid) AS n_tup_del, pg_stat_get_tuples_hot_updated(c.oid) AS n_tup_hot_upd, pg_stat_get_live_tuples(c.oid) AS n_live_tup, pg_stat_get_dead_tuples(c.oid) AS n_dead_tup, pg_stat_get_mod_since_analyze(c.oid) AS n_mod_since_analyze, pg_stat_get_last_vacuum_time(c.oid) AS last_vacuum, pg_stat_get_last_autovacuum_time(c.oid) AS last_autovacuum, pg_stat_get_last_analyze_time(c.oid) AS last_analyze, pg_stat_get_last_autoanalyze_time(c.oid) AS last_autoanalyze, pg_stat_get_vacuum_count(c.oid) AS vacuum_count, pg_stat_get_autovacuum_count(c.oid) AS autovacuum_count, pg_stat_get_analyze_count(c.oid) AS analyze_count, pg_stat_get_autoanalyze_count(c.oid) AS autoanalyze_count FROM pg_class c LEFT JOIN pg_index i ON c.oid = i.indrelid LEFT JOIN pg_namespace n ON n.oid = c.relnamespace WHERE c.relkind = ANY (ARRAY['r'::"char", 't'::"char", 'm'::"char"]) GROUP BY c.oid, n.nspname, c.relname;我们以n_live_tup这一列为例，看下这个统计信息来自哪里。需要梳理下pg_stat_get_live_tuples函数：可以看到统计信息来自pg_stat_tmp/global.stat（由配置项pgstat_temp_directory控制，默认pg_stat_tmp）
2、正常关闭服务
关闭服务前，统计信息文件位于pg_stat_tmp：
关闭服务后：统计信息文件移动到了pg_stat
3、再次正常启动服务
发现统计信息文件又移动到了，pg_stat_tmp。此时查看pg_stat_all_tables视图，发现统计信息是存在的：
为什么重启后统计信息并没有重置呢？
经分析，在崩溃重启恢复时，会将pg_stat_tmp和pg_stat目录都进行重置，此后重启成功后，因没有统计信息文件，就导致pg_stat_all_tables视图中的信息为空了：
