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doc: Clarify inherited constraint behavior

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Update the table inheritance documentation to mention not-null constraints
alongside check constraints where inherited constraints are discussed.

Also clarify that some properties of inherited constraints can now be altered
directly on child tables, while the resulting constraint must remain compatible
with its inherited parent constraints.  For multiple inheritance, say explicitly
that when a column or constraint is inherited from more than one parent, the
stricter definition applies.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/E74C57FA-1DD0-4C8E-8FB1-538034752592@gmail.com
This commit is contained in:
Andrew Dunstan 2026-06-27 17:43:02 -04:00
parent 0cd17fdd3c
commit d16be8605f
1 changed files with 19 additions and 14 deletions
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33
doc/src/sgml/ddl.sgml
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@ -4090,7 +4090,9 @@ VALUES ('Albany', NULL, NULL, 'NY');
similar fashion. Thus, for example, a merged column will be marked
not-null if any one of the column definitions it came from is marked
not-null. Check constraints are merged if they have the same name,
and the merge will fail if their conditions are different.
and the merge will fail if their conditions are different. For merged
check constraints, stricter enforceability is preserved: if any inherited
copy is enforced, the merged constraint is enforced.
</para>
<para>
@ -4104,8 +4106,9 @@ VALUES ('Albany', NULL, NULL, 'NY');
To do this the new child table must already include columns with
the same names and types as the columns of the parent. It must also include
check constraints with the same names and check expressions as those of the
parent. Similarly an inheritance link can be removed from a child using the
<literal>NO INHERIT</literal> variant of <command>ALTER TABLE</command>.
parent, as well as matching not-null constraints. Similarly an inheritance
link can be removed from a child using the <literal>NO INHERIT</literal>
variant of <command>ALTER TABLE</command>.
Dynamically adding and removing inheritance links like this can be useful
when the inheritance relationship is being used for table
partitioning (see <xref linkend="ddl-partitioning"/>).
@ -4124,21 +4127,23 @@ VALUES ('Albany', NULL, NULL, 'NY');
<para>
A parent table cannot be dropped while any of its children remain. Neither
can columns or check constraints of child tables be dropped or altered
if they are inherited
from any parent tables. If you wish to remove a table and all of its
descendants, one easy way is to drop the parent table with the
<literal>CASCADE</literal> option (see <xref linkend="ddl-depend"/>).
can inherited columns or inherited check and not-null constraints of child
tables be dropped directly. Some properties of inherited constraints can
be altered, but each resulting constraint must remain compatible with all
parent constraints from which it is inherited. If you wish to remove a
table and all of its descendants, one easy way is to drop the parent table
with the <literal>CASCADE</literal> option (see <xref linkend="ddl-depend"/>).
</para>
<para>
<command>ALTER TABLE</command> will
propagate any changes in column data definitions and check
constraints down the inheritance hierarchy. Again, dropping
columns that are depended on by other tables is only possible when using
the <literal>CASCADE</literal> option. <command>ALTER
TABLE</command> follows the same rules for duplicate column merging
and rejection that apply during <command>CREATE TABLE</command>.
propagate changes in column definitions and in inheritable constraints
(check and not-null constraints) down the inheritance hierarchy. Again,
dropping columns that are depended on by other tables is only possible
when using the <literal>CASCADE</literal> option. <command>ALTER
TABLE</command> follows the same rules for merging or rejecting duplicate
inherited column and constraint definitions that apply during
<command>CREATE TABLE</command>.
</para>
<para>