历史版本。 PostgreSQL 9.1 已结束支持。 2016-10-27. 请参阅 当前版本手册.
45.64. pg_stats #
视图pg_stats提供对存储在pg_statistic目录中信息的访问。此视图仅允许访问用户具有读取权限的表对应的pg_statistic行，因此可以安全地允许对此视图进行公共读取访问。
pg_stats也旨在以比底层目录更易读的格式呈现信息—
但其模式必须在为pg_statistic定义新的槽类型时进行扩展。
表 45.65. pg_stats 列
| Name | Type | References | Description |
|---|---|---|---|
schemaname | name | | Name of schema containing table |
tablename | name | | Name of table |
attname | name | | Name of the column described by this row |
inherited | bool | If true, this row includes inheritance child columns, not just the values in the specified table | |
null_frac | real | Fraction of column entries that are null | |
avg_width | integer | Average width in bytes of column's entries | |
n_distinct | real | If greater than zero, the estimated number of distinct values in the
column. If less than zero, the negative of the number of distinct
values divided by the number of rows. (The negated form is used when
ANALYZE believes that the number of distinct values is
likely to increase as the table grows; the positive form is used when
the column seems to have a fixed number of possible values.) For
example, -1 indicates a unique column in which the number of distinct
values is the same as the number of rows.
| |
most_common_vals | anyarray | A list of the most common values in the column. (Null if
no values seem to be more common than any others.)
For some data types such as tsvector, this is a list of
the most common element values rather than values of the type itself.
| |
most_common_freqs | real[] | A list of the frequencies of the most common values or elements,
i.e., number of occurrences of each divided by total number of rows.
(Null when most_common_vals is.)
For some data types such as tsvector, it can also store some
additional information, making it longer than the
most_common_vals array.
| |
histogram_bounds | anyarray | A list of values that divide the column's values into groups of
approximately equal population. The values in
most_common_vals, if present, are omitted from this
histogram calculation. (This column is null if the column data type
does not have a < operator or if the
most_common_vals list accounts for the entire
population.)
| |
correlation | real | Statistical correlation between physical row ordering and
logical ordering of the column values. This ranges from -1 to +1.
When the value is near -1 or +1, an index scan on the column will
be estimated to be cheaper than when it is near zero, due to reduction
of random access to the disk. (This column is null if the column data
type does not have a < operator.)
|
most_common_vals 和 histogram_bounds 数组中的最大项数可以逐列地用 ALTER TABLE SET STATISTICS 命令设置，或者全局地通过设置default_statistics_target运行时参数来设置。