diff options
Diffstat (limited to 'src/include')
| -rw-r--r-- | src/include/catalog/pg_statistic.h | 15 |
1 files changed, 8 insertions, 7 deletions
diff --git a/src/include/catalog/pg_statistic.h b/src/include/catalog/pg_statistic.h index 5d775a94fda..c1a624d979a 100644 --- a/src/include/catalog/pg_statistic.h +++ b/src/include/catalog/pg_statistic.h @@ -57,13 +57,14 @@ CATALOG(pg_statistic,2619) BKI_WITHOUT_OIDS * > 0 actual number of distinct values * < 0 negative of multiplier for number of rows * The special negative case allows us to cope with columns that are - * unique (stadistinct = -1) or nearly so (for example, a column in - * which values appear about twice on the average could be represented - * by stadistinct = -0.5). Because the number-of-rows statistic in - * pg_class may be updated more frequently than pg_statistic is, it's - * important to be able to describe such situations as a multiple of - * the number of rows, rather than a fixed number of distinct values. - * But in other cases a fixed number is correct (eg, a boolean column). + * unique (stadistinct = -1) or nearly so (for example, a column in which + * non-null values appear about twice on the average could be represented + * by stadistinct = -0.5 if there are no nulls, or -0.4 if 20% of the + * column is nulls). Because the number-of-rows statistic in pg_class may + * be updated more frequently than pg_statistic is, it's important to be + * able to describe such situations as a multiple of the number of rows, + * rather than a fixed number of distinct values. But in other cases a + * fixed number is correct (eg, a boolean column). * ---------------- */ float4 stadistinct; |