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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。
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