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Doc: clarify NULLS NOT DISTINCT use in unique indexes

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indexes-unique.html mentioned nothing about the availability of NULLS NOT
DISTINCT to modify the NULLs-are-not-equal behavior of unique indexes.
Add this to the synopsis and clarify what it does regarding NULLs.

Author: David Gilman, David Rowley
Reviewed-by: Corey Huinker
Discussion: https://postgr.es/m/CALBH9DDr3NLqzWop1z5uZE-M5G_GYUuAeHFHQeyzFbNd8W0d=Q@mail.gmail.com
Backpatch-through: 15, where NULLS NOT DISTINCT was added
This commit is contained in:
David Rowley
2023-04-20 23:52:36 +12:00
parent 62b22caa55
commit 94d73f9abd
1 changed files with 6 additions and 4 deletions
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10
doc/src/sgml/indices.sgml
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@@ -664,16 +664,18 @@ CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST);
Indexes can also be used to enforce uniqueness of a column's value,
or the uniqueness of the combined values of more than one column.
<synopsis>
CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>);
CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>) <optional> NULLS <optional> NOT </optional> DISTINCT </optional>;
</synopsis>
Currently, only B-tree indexes can be declared unique.
</para>
<para>
When an index is declared unique, multiple table rows with equal
indexed values are not allowed. Null values are not considered
equal. A multicolumn unique index will only reject cases where all
indexed columns are equal in multiple rows.
indexed values are not allowed. By default, null values in a unique column
are not considered equal, allowing multiple nulls in the column. The
<literal>NULLS NOT DISTINCT</literal> option modifies this and causes the
index to treat nulls as equal. A multicolumn unique index will only reject
cases where all indexed columns are equal in multiple rows.
</para>
<para>