postgres.git / summary / log / commit / refs
commit 3753e2720a9691d38aa85fe1e641ba55703e2a6f
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat Oct 17 20:02:47 2020 +0000
First released in: 14.0, 9.5.24, 9.6.20, 10.15, 11.10, 12.5, 13.1
Doc: caution against misuse of 'now' and related datetime literals.
Section 8.5.1.4, which defines these literals, made only a vague
reference to the fact that they might be evaluated too soon to be
safe in non-interactive contexts. Provide a more explicit caution
against misuse. Also, generalize the wording in the related tip in
section 9.9.4: while it clearly described this problem, it implied
(or really, stated outright) that the problem only applies to table
DEFAULT clauses.
Per gripe from Tijs van Dam. Back-patch to all supported branches.
Discussion: https://postgr.es/m/c2LuRv9BiRT3bqIo5mMQiVraEXey_25B4vUn0kDqVqilwOEu_iVF1tbtvLnyQK7yDG3PFaz_GxLLPil2SDkj1MCObNRVaac-7j1dVdFERk8=@thalex.com
doc/src/sgml/datatype.sgml | 20 +++++++++++++++++---
doc/src/sgml/func.sgml | 8 +++++---
2 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 9bf26ffcfd7..bcc013ddbdf 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -2114,7 +2114,7 @@ January 8 04:05:06 1999 PST
</para>
</sect3>
- <sect3>
+ <sect3 id="datatype-datetime-special-values">
<title>Special Values</title>
<indexterm>
@@ -2202,12 +2202,26 @@ January 8 04:05:06 1999 PST
type:
<literal>CURRENT_DATE</literal>, <literal>CURRENT_TIME</literal>,
<literal>CURRENT_TIMESTAMP</literal>, <literal>LOCALTIME</literal>,
- <literal>LOCALTIMESTAMP</literal>. The latter four accept an
- optional subsecond precision specification. (See <xref
+ <literal>LOCALTIMESTAMP</literal>. (See <xref
linkend="functions-datetime-current"/>.) Note that these are
SQL functions and are <emphasis>not</emphasis> recognized in data input strings.
</para>
+ <caution>
+ <para>
+ While the input strings <literal>now</literal>,
+ <literal>today</literal>, <literal>tomorrow</literal>,
+ and <literal>yesterday</literal> are fine to use in interactive SQL
+ commands, they can have surprising behavior when the command is
+ saved to be executed later, for example in prepared statements,
+ views, and function definitions. The string can be converted to a
+ specific time value that continues to be used long after it becomes
+ stale. Use one of the SQL functions instead in such contexts.
+ For example, <literal>CURRENT_DATE + 1</literal> is safer than
+ <literal>'tomorrow'::date</literal>.
+ </para>
+ </caution>
+
</sect3>
</sect2>
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index a698f424dee..9e5b2deb72f 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -8727,20 +8727,22 @@ now()
<programlisting>
SELECT CURRENT_TIMESTAMP;
SELECT now();
-SELECT TIMESTAMP 'now'; -- incorrect for use with DEFAULT
+SELECT TIMESTAMP 'now'; -- but see tip below
</programlisting>
</para>
<tip>
<para>
- You do not want to use the third form when specifying a <literal>DEFAULT</literal>
- clause while creating a table. The system will convert <literal>now</literal>
+ Do not use the third form when specifying a value to be evaluated later,
+ for example in a <literal>DEFAULT</literal> clause for a table column.
+ The system will convert <literal>now</literal>
to a <type>timestamp</type> as soon as the constant is parsed, so that when
the default value is needed,
the time of the table creation would be used! The first two
forms will not be evaluated until the default value is used,
because they are function calls. Thus they will give the desired
behavior of defaulting to the time of row insertion.
+ (See also <xref linkend="datatype-datetime-special-values"/>.)
</para>
</tip>
</sect2>
[parent: b39c94097da9]