Movatterモバイル変換

[0]ホーム
URL:

Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly

 Sign up
Appearance settings

Commit1f66c65

Browse files
committed
Doc: document that we expect CHECK constraint conditions to be immutable.
This restriction is implicit in the check-only-once implementation we usefor table and domain constraints, but it wasn't spelled out anywhere, norwas there any advice about how to alter a constraint's behavior safely.Improve that.I was also dissatisfied with the documentation of ALTER DOMAIN VALIDATECONSTRAINT, which entirely failed to explain the use of that feature; andthence decided that ALTER TABLE VALIDATE CONSTRAINT could be documentedbetter as well.Perhaps we should back-patch this, along with the related commit36d442a,but for now I refrained.Discussion:https://postgr.es/m/12539.1544107316@sss.pgh.pa.us
1 parent1464755 commit1f66c65

File tree

4 files changed

+123
-33
lines changed

4 files changed

+123
-33
lines changed

‎doc/src/sgml/ddl.sgml

Lines changed: 27 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -427,7 +427,33 @@ CREATE TABLE products (
427427
guarantee, a custom <link linkend="triggers">trigger</link> can be used
428428
to implement that. (This approach avoids the dump/reload problem because
429429
<application>pg_dump</application> does not reinstall triggers until after
430-
reloading data, so that the check will not be enforced during a dump/reload.)
430+
reloading data, so that the check will not be enforced during a
431+
dump/reload.)
432+
</para>
433+
</note>
434+
435+
<note>
436+
<para>
437+
<productname>PostgreSQL</productname> assumes that
438+
<literal>CHECK</literal> constraints' conditions are immutable, that
439+
is, they will always give the same result for the same input row.
440+
This assumption is what justifies examining <literal>CHECK</literal>
441+
constraints only when rows are inserted or updated, and not at other
442+
times. (The warning above about not referencing other table data is
443+
really a special case of this restriction.)
444+
</para>
445+
446+
<para>
447+
An example of a common way to break this assumption is to reference a
448+
user-defined function in a <literal>CHECK</literal> expression, and
449+
then change the behavior of that
450+
function. <productname>PostgreSQL</productname> does not disallow
451+
that, but it will not notice if there are rows in the table that now
452+
violate the <literal>CHECK</literal> constraint. That would cause a
453+
subsequent database dump and reload to fail.
454+
The recommended way to handle such a change is to drop the constraint
455+
(using <command>ALTER TABLE</command>), adjust the function definition,
456+
and re-add the constraint, thereby rechecking it against all table rows.
431457
</para>
432458
</note>
433459
</sect2>

‎doc/src/sgml/ref/alter_domain.sgml

Lines changed: 18 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -118,8 +118,8 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
118118
<listitem>
119119
<para>
120120
This form validates a constraint previously added as
121-
<literal>NOT VALID</literal>, that is,verifythat alldata in columns using the
122-
domain satisfy the specified constraint.
121+
<literal>NOT VALID</literal>, that is,it verifiesthat allvalues in
122+
table columns of thedomain type satisfy the specified constraint.
123123
</para>
124124
</listitem>
125125
</varlistentry>
@@ -202,7 +202,7 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
202202
<term><literal>NOT VALID</literal></term>
203203
<listitem>
204204
<para>
205-
Do not verify existingcolumn data for constraint validity.
205+
Do not verify existingstored data for constraint validity.
206206
</para>
207207
</listitem>
208208
</varlistentry>
@@ -272,6 +272,21 @@ ALTER DOMAIN <replaceable class="parameter">name</replaceable>
272272
<refsect1>
273273
<title>Notes</title>
274274

275+
<para>
276+
Although <command>ALTER DOMAIN ADD CONSTRAINT</command> attempts to verify
277+
that existing stored data satisfies the new constraint, this check is not
278+
bulletproof, because the command cannot <quote>see</quote> table rows that
279+
are newly inserted or updated and not yet committed. If there is a hazard
280+
that concurrent operations might insert bad data, the way to proceed is to
281+
add the constraint using the <literal>NOT VALID</literal> option, commit
282+
that command, wait until all transactions started before that commit have
283+
finished, and then issue <command>ALTER DOMAIN VALIDATE
284+
CONSTRAINT</command> to search for data violating the constraint. This
285+
method is reliable because once the constraint is committed, all new
286+
transactions are guaranteed to enforce it against new values of the domain
287+
type.
288+
</para>
289+
275290
<para>
276291
Currently, <command>ALTER DOMAIN ADD CONSTRAINT</command>, <command>ALTER
277292
DOMAIN VALIDATE CONSTRAINT</command>, and <command>ALTER DOMAIN SET NOT

‎doc/src/sgml/ref/alter_table.sgml

Lines 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
358358
<xref linkend="sql-createtable"/>, plus the option <literal>NOT
359359
VALID</literal>, which is currently only allowed for foreign key
360360
and CHECK constraints.
361-
If the constraint is marked <literal>NOT VALID</literal>, the
362-
potentially-lengthy initial check to verify that all rows in the table
363-
satisfy the constraint is skipped. The constraint will still be
361+
</para>
362+
363+
<para>
364+
Normally, this form will cause a scan of the table to verify that all
365+
existing rows in the table satisfy the new constraint. But if
366+
the <literal>NOT VALID</literal> option is used, this
367+
potentially-lengthy scan is skipped. The constraint will still be
364368
enforced against subsequent inserts or updates (that is, they'll fail
365369
unless there is a matching row in the referenced table, in the case
366-
of foreign keys; and they'll fail unless the new row matches the
367-
specified checkconstraints). But the
370+
of foreign keys, or they'll fail unless the new row matches the
371+
specified checkcondition). But the
368372
database will not assume that the constraint holds for all rows in
369373
the table, until it is validated by using the <literal>VALIDATE
370-
CONSTRAINT</literal> option. Foreign key constraints on partitioned
371-
tables may not be declared <literal>NOT VALID</literal> at present.
374+
CONSTRAINT</literal> option.
375+
See <xref linkend="sql-altertable-notes"
376+
endterm="sql-altertable-notes-title"/> below for more information
377+
about using the <literal>NOT VALID</literal> option.
372378
</para>
373379

374380
<para>
375-
The addition of a foreign key constraint requires a
376-
<literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table.
381+
Addition of a foreign key constraint requires a
382+
<literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table,
383+
in addition to the lock on the table receiving the constraint.
377384
</para>
378385

379386
<para>
380387
Additional restrictions apply when unique or primary key constraints
381-
are added to partitioned tables; see <xref linkend="sql-createtable" />.
388+
are added to partitioned tables; see <xref linkend="sql-createtable"/>.
389+
Also, foreign key constraints on partitioned
390+
tables may not be declared <literal>NOT VALID</literal> at present.
382391
</para>
383392

384393
</listitem>
@@ -453,23 +462,13 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
453462
<term><literal>VALIDATE CONSTRAINT</literal></term>
454463
<listitem>
455464
<para>
456-
This form validates a foreign key or check constraint that was previously created
457-
as <literal>NOT VALID</literal>, by scanning the table to ensure there
458-
are no rows for which the constraint is not satisfied.
459-
Nothing happens if the constraint is already marked valid.
460-
</para>
461-
<para>
462-
Validation can be a long process on larger tables. The value of separating
463-
validation from initial creation is that you can defer validation to less
464-
busy times, or can be used to give additional time to correct pre-existing
465-
errors while preventing new errors. Note also that validation on its own
466-
does not prevent normal write commands against the table while it runs.
467-
</para>
468-
<para>
469-
Validation acquires only a <literal>SHARE UPDATE EXCLUSIVE</literal> lock
470-
on the table being altered. If the constraint is a foreign key then
471-
a <literal>ROW SHARE</literal> lock is also required on
472-
the table referenced by the constraint.
465+
This form validates a foreign key or check constraint that was
466+
previously created as <literal>NOT VALID</literal>, by scanning the
467+
table to ensure there are no rows for which the constraint is not
468+
satisfied. Nothing happens if the constraint is already marked valid.
469+
(See <xref linkend="sql-altertable-notes"
470+
endterm="sql-altertable-notes-title"/> below for an explanation of the
471+
usefulness of this command.)
473472
</para>
474473
</listitem>
475474
</varlistentry>
@@ -1153,8 +1152,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
11531152
</variablelist>
11541153
</refsect1>
11551154

1156-
<refsect1>
1157-
<title>Notes</title>
1155+
<refsect1 id="sql-altertable-notes">
1156+
<title id="sql-altertable-notes-title">Notes</title>
11581157

11591158
<para>
11601159
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.
12001199
</para>
12011200

1201+
<para>
1202+
Scanning a large table to verify a new foreign key or check constraint
1203+
can take a long time, and other updates to the table are locked out
1204+
until the <command>ALTER TABLE ADD CONSTRAINT</command> command is
1205+
committed. The main purpose of the <literal>NOT VALID</literal>
1206+
constraint option is to reduce the impact of adding a constraint on
1207+
concurrent updates. With <literal>NOT VALID</literal>,
1208+
the <command>ADD CONSTRAINT</command> command does not scan the table
1209+
and can be committed immediately. After that, a <literal>VALIDATE
1210+
CONSTRAINT</literal> command can be issued to verify that existing rows
1211+
satisfy the constraint. The validation step does not need to lock out
1212+
concurrent updates, since it knows that other transactions will be
1213+
enforcing the constraint for rows that they insert or update; only
1214+
pre-existing rows need to be checked. Hence, validation acquires only
1215+
a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the table being
1216+
altered. (If the constraint is a foreign key then a <literal>ROW
1217+
SHARE</literal> lock is also required on the table referenced by the
1218+
constraint.) In addition to improving concurrency, it can be useful to
1219+
use <literal>NOT VALID</literal> and <literal>VALIDATE
1220+
CONSTRAINT</literal> in cases where the table is known to contain
1221+
pre-existing violations. Once the constraint is in place, no new
1222+
violations can be inserted, and the existing problems can be corrected
1223+
at leisure until <literal>VALIDATE CONSTRAINT</literal> finally
1224+
succeeds.
1225+
</para>
1226+
12021227
<para>
12031228
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

Lines 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));
214214
and then to apply column <literal>NOT NULL</literal> constraints to columns of
215215
the domain type as needed, rather than directly to the domain type.
216216
</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
225+
of table <literal>CHECK</literal> constraints, as described in
226+
<xref linkend="ddl-constraints-check-constraints"/>.)
227+
</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
233+
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>
218242

219243
<refsect1>

0 commit comments

Comments
 (0)
[8]ページ先頭
©2009-2025 Movatter.jp