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Add UNIQUE null treatment option

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The SQL standard has been ambiguous about whether null values in
unique constraints should be considered equal or not.  Different
implementations have different behaviors.  In the SQL:202x draft, this
has been formalized by making this implementation-defined and adding
an option on unique constraint definitions UNIQUE [ NULLS [NOT]
DISTINCT ] to choose a behavior explicitly.

This patch adds this option to PostgreSQL.  The default behavior
remains UNIQUE NULLS DISTINCT.  Making this happen in the btree code
is pretty easy; most of the patch is just to carry the flag around to
all the places that need it.

The CREATE UNIQUE INDEX syntax extension is not from the standard,
it's my own invention.

I named all the internal flags, catalog columns, etc. in the negative
("nulls not distinct") so that the default PostgreSQL behavior is the
default if the flag is false.

Reviewed-by: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/84e5ee1b-387e-9a54-c326-9082674bde78@enterprisedb.com
This commit is contained in:
Peter Eisentraut
2022-02-03 11:29:54 +01:00
parent f862d57057
commit 94aa7cc5f7
36 changed files with 348 additions and 57 deletions
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13
doc/src/sgml/catalogs.sgml
View File

@ -4265,6 +4265,19 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>indnullsnotdistinct</structfield> <type>bool</type>
</para>
<para>
This value is only used for unique indexes. If false, this unique
index will consider null values distinct (so the index can contain
multiple null values in a column, the default PostgreSQL behavior). If
it is true, it will consider null values to be equal (so the index can
only contain one null value in a column).
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>indisprimary</structfield> <type>bool</type>

29
doc/src/sgml/ddl.sgml
View File

@ -759,14 +759,33 @@ CREATE TABLE products (
In general, a unique constraint is violated if there is more than
one row in the table where the values of all of the
columns included in the constraint are equal.
However, two null values are never considered equal in this
By default, two null values are not considered equal in this
comparison. That means even in the presence of a
unique constraint it is possible to store duplicate
rows that contain a null value in at least one of the constrained
columns. This behavior conforms to the SQL standard, but we have
heard that other SQL databases might not follow this rule. So be
careful when developing applications that are intended to be
portable.
columns. This behavior can be changed by adding the clause <literal>NULLS
NOT DISTINCT</literal>, like
<programlisting>
CREATE TABLE products (
product_no integer UNIQUE <emphasis>NULLS NOT DISTINCT</emphasis>,
name text,
price numeric
);
</programlisting>
or
<programlisting>
CREATE TABLE products (
product_no integer,
name text,
price numeric,
UNIQUE <emphasis>NULLS NOT DISTINCT</emphasis> (product_no)
);
</programlisting>
The default behavior can be specified explicitly using <literal>NULLS
DISTINCT</literal>. The default null treatment in unique constraints is
implementation-defined according to the SQL standard, and other
implementations have a different behavior. So be careful when developing
applications that are intended to be portable.
</para>
</sect2>

12
doc/src/sgml/information_schema.sgml
View File

@ -6899,6 +6899,18 @@ ORDER BY c.ordinal_position;
<literal>YES</literal>)
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>nulls_distinct</structfield> <type>yes_or_no</type>
</para>
<para>
If the constraint is a unique constraint, then <literal>YES</literal>
if the constraint treats nulls as distinct or <literal>NO</literal> if
it treats nulls as not distinct, otherwise null for other types of
constraints.
</para></entry>
</row>
</tbody>
</tgroup>
</table>

4
doc/src/sgml/ref/alter_table.sgml
View File

@ -103,7 +103,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
DEFAULT <replaceable>default_expr</replaceable> |
GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
@ -113,7 +113,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
{ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]

13
doc/src/sgml/ref/create_index.sgml
View File

@ -24,6 +24,7 @@ PostgreSQL documentation
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> ] ON [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
( { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">opclass</replaceable> [ ( <replaceable class="parameter">opclass_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
[ NULLS [ NOT ] DISTINCT ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
[ TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> ]
[ WHERE <replaceable class="parameter">predicate</replaceable> ]
@ -334,6 +335,18 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
</listitem>
</varlistentry>
<varlistentry>
<term><literal>NULLS DISTINCT</literal></term>
<term><literal>NULLS NOT DISTINCT</literal></term>
<listitem>
<para>
Specifies whether for a unique index, null values should be considered
distinct (not equal). The default is that they are distinct, so that
a unique index could contain multiple null values in a column.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">storage_parameter</replaceable></term>
<listitem>

11
doc/src/sgml/ref/create_table.sgml
View File

@ -67,7 +67,7 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
DEFAULT <replaceable>default_expr</replaceable> |
GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
@ -77,7 +77,7 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
{ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
@ -917,8 +917,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
</varlistentry>
<varlistentry>
<term><literal>UNIQUE</literal> (column constraint)</term>
<term><literal>UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
<term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ]</literal> (column constraint)</term>
<term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
<optional> <literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>
<listitem>
@ -934,7 +934,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
<para>
For the purpose of a unique constraint, null values are not
considered equal.
considered equal, unless <literal>NULLS NOT DISTINCT</literal> is
specified.
</para>
<para>