PG统计信息之数据字典

pg_stats 优化器统计信息视图，通过用于计算selectivity，估算匹配条件行，用于辅助优化器选择合适访问路径，连接方式

\d pg_stats
                     View "pg_catalog.pg_stats"
         Column         |   Type   | Collation | Nullable | Default 
------------------------+----------+-----------+----------+---------
 schemaname             | name     |           |          | 
 tablename              | name     |           |          | 
 attname                | name     |           |          | 
 inherited              | boolean  |           |          | 
 null_frac              | real     |           |          | 
 avg_width              | integer  |           |          | 
 n_distinct             | real     |           |          | 
 most_common_vals       | anyarray |           |          | 
 most_common_freqs      | real[]   |           |          | 
 histogram_bounds       | anyarray |           |          | 
 correlation            | real     |           |          | 
 most_common_elems      | anyarray |           |          | 
 most_common_elem_freqs | real[]   |           |          | 
 elem_count_histogram   | real[]   |           |          | 
 

null_frac ：列中为空的比例

n_distinct：该列的唯一值个数

most_common_vals：高频值

most_common_freqs ：高频值占比

histogram_bounds：排除高频值之后的平均分布区间（通常分10个区间，10%）

correlation：表的物理存储顺序和列值分布的相关性

most_common_elems：数组类型统计信息：数组中的高频元素

most_common_elem_freqs：高频元素占比

elem_count_histogram：该列元素唯一值个数平均分布柱状图

PG使用histogram_bounds 记录频率等高，mcv/mcf标注数据倾斜 频率，兼顾大部分数据平均分布，少部分数据倾斜的高频值

pg_class 记载表和几乎所有有字段或者是那些类似表的东西

\d pg_class
                     Table "pg_catalog.pg_class"
       Column        |     Type     | Collation | Nullable | Default
---------------------+--------------+-----------+----------+---------
 oid                 | oid          |           | not null |
 relname             | name         |           | not null |
 relnamespace        | oid          |           | not null |
 reltype             | oid          |           | not null |
 reloftype           | oid          |           | not null |
 relowner            | oid          |           | not null |
 relam               | oid          |           | not null |
 relfilenode         | oid          |           | not null |
 reltablespace       | oid          |           | not null |
 relpages            | integer      |           | not null |
 reltuples           | real         |           | not null |
 relallvisible       | integer      |           | not null |
 reltoastrelid       | oid          |           | not null |
 relhasindex         | boolean      |           | not null |
 relisshared         | boolean      |           | not null |
 relpersistence      | "char"       |           | not null |
 relkind             | "char"       |           | not null |
 relnatts            | smallint     |           | not null |
 relchecks           | smallint     |           | not null |
 relhasrules         | boolean      |           | not null |
 relhastriggers      | boolean      |           | not null |
 relhassubclass      | boolean      |           | not null |
 relrowsecurity      | boolean      |           | not null |
 relforcerowsecurity | boolean      |           | not null |
 relispopulated      | boolean      |           | not null |
 relreplident        | "char"       |           | not null |
 relispartition      | boolean      |           | not null |
 relrewrite          | oid          |           | not null |
 relfrozenxid        | xid          |           | not null |
 relminmxid          | xid          |           | not null |
 relacl              | aclitem[]    |           |          |
 reloptions          | text[]       | C         |          |
 relpartbound        | pg_node_tree | C         |          |

relpages 页数

reltuples 表中行的数目

relallvisible 表的可见映射中标记所有可见的页的数