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Commit 1f66c65

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Doc: document that we expect CHECK constraint conditions to be immutable.
This restriction is implicit in the check-only-once implementation we use for table and domain constraints, but it wasn't spelled out anywhere, nor was there any advice about how to alter a constraint's behavior safely. Improve that. I was also dissatisfied with the documentation of ALTER DOMAIN VALIDATE CONSTRAINT, which entirely failed to explain the use of that feature; and thence decided that ALTER TABLE VALIDATE CONSTRAINT could be documented better as well. Perhaps we should back-patch this, along with the related commit 36d442a, but for now I refrained. Discussion: https://postgr.es/m/12539.1544107316@sss.pgh.pa.us
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‎doc/src/sgml/ddl.sgml

Copy file name to clipboardExpand all lines: doc/src/sgml/ddl.sgml
+27-1Lines changed: 27 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -427,7 +427,33 @@ CREATE TABLE products (
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guarantee, a custom <link linkend="triggers">trigger</link> can be used
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to implement that. (This approach avoids the dump/reload problem because
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<application>pg_dump</application> does not reinstall triggers until after
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reloading data, so that the check will not be enforced during a dump/reload.)
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reloading data, so that the check will not be enforced during a
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dump/reload.)
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</para>
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</note>
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<note>
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<para>
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<productname>PostgreSQL</productname> assumes that
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<literal>CHECK</literal> constraints' conditions are immutable, that
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is, they will always give the same result for the same input row.
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This assumption is what justifies examining <literal>CHECK</literal>
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constraints only when rows are inserted or updated, and not at other
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times. (The warning above about not referencing other table data is
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really a special case of this restriction.)
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</para>
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<para>
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An example of a common way to break this assumption is to reference a
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user-defined function in a <literal>CHECK</literal> expression, and
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then change the behavior of that
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function. <productname>PostgreSQL</productname> does not disallow
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that, but it will not notice if there are rows in the table that now
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violate the <literal>CHECK</literal> constraint. That would cause a
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subsequent database dump and reload to fail.
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The recommended way to handle such a change is to drop the constraint
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(using <command>ALTER TABLE</command>), adjust the function definition,
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and re-add the constraint, thereby rechecking it against all table rows.
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</para>
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</note>
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</sect2>

‎doc/src/sgml/ref/alter_domain.sgml

Copy file name to clipboardExpand all lines: doc/src/sgml/ref/alter_domain.sgml
+18-3Lines changed: 18 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -118,8 +118,8 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
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<listitem>
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<para>
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This form validates a constraint previously added as
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<literal>NOT VALID</literal>, that is, verify that all data in columns using the
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domain satisfy the specified constraint.
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<literal>NOT VALID</literal>, that is, it verifies that all values in
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table columns of the domain type satisfy the specified constraint.
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</para>
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</listitem>
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</varlistentry>
@@ -202,7 +202,7 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
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<term><literal>NOT VALID</literal></term>
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<listitem>
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<para>
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Do not verify existing column data for constraint validity.
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Do not verify existing stored data for constraint validity.
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</para>
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</listitem>
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</varlistentry>
@@ -272,6 +272,21 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
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<refsect1>
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<title>Notes</title>
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<para>
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Although <command>ALTER DOMAIN ADD CONSTRAINT</command> attempts to verify
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that existing stored data satisfies the new constraint, this check is not
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bulletproof, because the command cannot <quote>see</quote> table rows that
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are newly inserted or updated and not yet committed. If there is a hazard
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that concurrent operations might insert bad data, the way to proceed is to
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add the constraint using the <literal>NOT VALID</literal> option, commit
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that command, wait until all transactions started before that commit have
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finished, and then issue <command>ALTER DOMAIN VALIDATE
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CONSTRAINT</command> to search for data violating the constraint. This
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method is reliable because once the constraint is committed, all new
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transactions are guaranteed to enforce it against new values of the domain
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type.
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</para>
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<para>
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Currently, <command>ALTER DOMAIN ADD CONSTRAINT</command>, <command>ALTER
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DOMAIN VALIDATE CONSTRAINT</command>, and <command>ALTER DOMAIN SET NOT

‎doc/src/sgml/ref/alter_table.sgml

Copy file name to clipboardExpand all lines: doc/src/sgml/ref/alter_table.sgml
+54-29Lines changed: 54 additions & 29 deletions
Original file line numberDiff line numberDiff line change
@@ -358,27 +358,36 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
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<xref linkend="sql-createtable"/>, plus the option <literal>NOT
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VALID</literal>, which is currently only allowed for foreign key
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and CHECK constraints.
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If the constraint is marked <literal>NOT VALID</literal>, the
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potentially-lengthy initial check to verify that all rows in the table
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satisfy the constraint is skipped. The constraint will still be
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</para>
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<para>
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Normally, this form will cause a scan of the table to verify that all
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existing rows in the table satisfy the new constraint. But if
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the <literal>NOT VALID</literal> option is used, this
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potentially-lengthy scan is skipped. The constraint will still be
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enforced against subsequent inserts or updates (that is, they'll fail
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unless there is a matching row in the referenced table, in the case
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of foreign keys; and they'll fail unless the new row matches the
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specified check constraints). But the
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of foreign keys, or they'll fail unless the new row matches the
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specified check condition). But the
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database will not assume that the constraint holds for all rows in
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the table, until it is validated by using the <literal>VALIDATE
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CONSTRAINT</literal> option. Foreign key constraints on partitioned
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tables may not be declared <literal>NOT VALID</literal> at present.
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CONSTRAINT</literal> option.
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See <xref linkend="sql-altertable-notes"
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endterm="sql-altertable-notes-title"/> below for more information
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about using the <literal>NOT VALID</literal> option.
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</para>
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<para>
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The addition of a foreign key constraint requires a
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<literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table.
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Addition of a foreign key constraint requires a
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<literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table,
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in addition to the lock on the table receiving the constraint.
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</para>
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<para>
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Additional restrictions apply when unique or primary key constraints
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are added to partitioned tables; see <xref linkend="sql-createtable" />.
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are added to partitioned tables; see <xref linkend="sql-createtable"/>.
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Also, foreign key constraints on partitioned
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tables may not be declared <literal>NOT VALID</literal> at present.
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</para>
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</listitem>
@@ -453,23 +462,13 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
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<term><literal>VALIDATE CONSTRAINT</literal></term>
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<listitem>
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<para>
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This form validates a foreign key or check constraint that was previously created
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as <literal>NOT VALID</literal>, by scanning the table to ensure there
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are no rows for which the constraint is not satisfied.
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Nothing happens if the constraint is already marked valid.
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</para>
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<para>
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Validation can be a long process on larger tables. The value of separating
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validation from initial creation is that you can defer validation to less
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busy times, or can be used to give additional time to correct pre-existing
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errors while preventing new errors. Note also that validation on its own
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does not prevent normal write commands against the table while it runs.
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</para>
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<para>
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Validation acquires only a <literal>SHARE UPDATE EXCLUSIVE</literal> lock
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on the table being altered. If the constraint is a foreign key then
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a <literal>ROW SHARE</literal> lock is also required on
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the table referenced by the constraint.
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This form validates a foreign key or check constraint that was
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previously created as <literal>NOT VALID</literal>, by scanning the
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table to ensure there are no rows for which the constraint is not
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satisfied. Nothing happens if the constraint is already marked valid.
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(See <xref linkend="sql-altertable-notes"
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endterm="sql-altertable-notes-title"/> below for an explanation of the
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usefulness of this command.)
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</para>
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</listitem>
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</varlistentry>
@@ -1153,8 +1152,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
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</variablelist>
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</refsect1>
11551154

1156-
<refsect1>
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<title>Notes</title>
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<refsect1 id="sql-altertable-notes">
1156+
<title id="sql-altertable-notes-title">Notes</title>
11581157

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<para>
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The key word <literal>COLUMN</literal> is noise and can be omitted.
@@ -1199,6 +1198,32 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
11991198
rewrites can thereby be combined into a single pass over the table.
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</para>
12011200

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<para>
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Scanning a large table to verify a new foreign key or check constraint
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can take a long time, and other updates to the table are locked out
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until the <command>ALTER TABLE ADD CONSTRAINT</command> command is
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committed. The main purpose of the <literal>NOT VALID</literal>
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constraint option is to reduce the impact of adding a constraint on
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concurrent updates. With <literal>NOT VALID</literal>,
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the <command>ADD CONSTRAINT</command> command does not scan the table
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and can be committed immediately. After that, a <literal>VALIDATE
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CONSTRAINT</literal> command can be issued to verify that existing rows
1211+
satisfy the constraint. The validation step does not need to lock out
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concurrent updates, since it knows that other transactions will be
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enforcing the constraint for rows that they insert or update; only
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pre-existing rows need to be checked. Hence, validation acquires only
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a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the table being
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altered. (If the constraint is a foreign key then a <literal>ROW
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SHARE</literal> lock is also required on the table referenced by the
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constraint.) In addition to improving concurrency, it can be useful to
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use <literal>NOT VALID</literal> and <literal>VALIDATE
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CONSTRAINT</literal> in cases where the table is known to contain
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pre-existing violations. Once the constraint is in place, no new
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violations can be inserted, and the existing problems can be corrected
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at leisure until <literal>VALIDATE CONSTRAINT</literal> finally
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succeeds.
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</para>
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12021227
<para>
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The <literal>DROP COLUMN</literal> form does not physically remove
12041229
the column, but simply makes it invisible to SQL operations. Subsequent

‎doc/src/sgml/ref/create_domain.sgml

Copy file name to clipboardExpand all lines: doc/src/sgml/ref/create_domain.sgml
+24Lines changed: 24 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -214,6 +214,30 @@ INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
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and then to apply column <literal>NOT NULL</literal> constraints to columns of
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the domain type as needed, rather than directly to the domain type.
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</para>
217+
218+
<para>
219+
<productname>PostgreSQL</productname> assumes that
220+
<literal>CHECK</literal> constraints' conditions are immutable, that is,
221+
they will always give the same result for the same input value. This
222+
assumption is what justifies examining <literal>CHECK</literal>
223+
constraints only when a value is first converted to be of a domain type,
224+
and not at other times. (This is essentially the same as the treatment
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of table <literal>CHECK</literal> constraints, as described in
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<xref linkend="ddl-constraints-check-constraints"/>.)
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</para>
228+
229+
<para>
230+
An example of a common way to break this assumption is to reference a
231+
user-defined function in a <literal>CHECK</literal> expression, and then
232+
change the behavior of that
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function. <productname>PostgreSQL</productname> does not disallow that,
234+
but it will not notice if there are stored values of the domain type that
235+
now violate the <literal>CHECK</literal> constraint. That would cause a
236+
subsequent database dump and reload to fail. The recommended way to
237+
handle such a change is to drop the constraint (using <command>ALTER
238+
DOMAIN</command>), adjust the function definition, and re-add the
239+
constraint, thereby rechecking it against stored data.
240+
</para>
217241
</refsect1>
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219243
<refsect1>

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