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Fix constant-folding of ROW(...) IS [NOT] NULL with composite fields.

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The SQL standard appears to specify that IS [NOT] NULL's tests of field
nullness are non-recursive, ie, we shouldn't consider that a composite
field with value ROW(NULL,NULL) is null for this purpose.
ExecEvalNullTest got this right, but eval_const_expressions did not,
leading to weird inconsistencies depending on whether the expression
was such that the planner could apply constant folding.

Also, adjust the docs to mention that IS [NOT] DISTINCT FROM NULL can be
used as a substitute test if a simple null check is wanted for a rowtype
argument.  That motivated reordering things so that IS [NOT] DISTINCT FROM
is described before IS [NOT] NULL.  In HEAD, I went a bit further and added
a table showing all the comparison-related predicates.

Per bug #14235.  Back-patch to all supported branches, since it's certainly
undesirable that constant-folding should change the semantics.

Report and patch by Andrew Gierth; assorted wordsmithing and revised
regression test cases by me.

Report: <20160708024746.1410.57282@wrigleys.postgresql.org>
This commit is contained in:
Tom Lane
2016-07-26 15:25:02 -04:00
parent 98b7a3cf2b
commit c235d510ea
5 changed files with 136 additions and 40 deletions
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70
doc/src/sgml/func.sgml
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@ -289,6 +289,32 @@
a nonempty range is always implied.
</para>
<para>
<indexterm>
<primary>IS DISTINCT FROM</primary>
</indexterm>
<indexterm>
<primary>IS NOT DISTINCT FROM</primary>
</indexterm>
Ordinary comparison operators yield null (signifying <quote>unknown</>),
not true or false, when either input is null. For example,
<literal>7 = NULL</> yields null, as does <literal>7 &lt;&gt; NULL</>. When
this behavior is not suitable, use the
<literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
<synopsis>
<replaceable>a</replaceable> IS DISTINCT FROM <replaceable>b</replaceable>
<replaceable>a</replaceable> IS NOT DISTINCT FROM <replaceable>b</replaceable>
</synopsis>
For non-null inputs, <literal>IS DISTINCT FROM</literal> is
the same as the <literal>&lt;&gt;</> operator. However, if both
inputs are null it returns false, and if only one input is
null it returns true. Similarly, <literal>IS NOT DISTINCT
FROM</literal> is identical to <literal>=</literal> for non-null
inputs, but it returns true when both inputs are null, and false when only
one input is null. Thus, these constructs effectively act as though null
were a normal data value, rather than <quote>unknown</>.
</para>
<para>
<indexterm>
<primary>IS NULL</primary>
@ -320,8 +346,7 @@
<literal><replaceable>expression</replaceable> = NULL</literal>
because <literal>NULL</> is not <quote>equal to</quote>
<literal>NULL</>. (The null value represents an unknown value,
and it is not known whether two unknown values are equal.) This
behavior conforms to the SQL standard.
and it is not known whether two unknown values are equal.)
</para>
<tip>
@ -338,7 +363,6 @@
</para>
</tip>
<note>
<para>
If the <replaceable>expression</replaceable> is row-valued, then
<literal>IS NULL</> is true when the row expression itself is null
@ -346,39 +370,13 @@
<literal>IS NOT NULL</> is true when the row expression itself is non-null
and all the row's fields are non-null. Because of this behavior,
<literal>IS NULL</> and <literal>IS NOT NULL</> do not always return
inverse results for row-valued expressions, i.e., a row-valued
expression that contains both NULL and non-null values will return false
for both tests.
This definition conforms to the SQL standard, and is a change from the
inconsistent behavior exhibited by <productname>PostgreSQL</productname>
versions prior to 8.2.
</para>
</note>
<para>
<indexterm>
<primary>IS DISTINCT FROM</primary>
</indexterm>
<indexterm>
<primary>IS NOT DISTINCT FROM</primary>
</indexterm>
Ordinary comparison operators yield null (signifying <quote>unknown</>),
not true or false, when either input is null. For example,
<literal>7 = NULL</> yields null, as does <literal>7 &lt;&gt; NULL</>. When
this behavior is not suitable, use the
<literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
<synopsis>
<replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>
<replaceable>expression</replaceable> IS NOT DISTINCT FROM <replaceable>expression</replaceable>
</synopsis>
For non-null inputs, <literal>IS DISTINCT FROM</literal> is
the same as the <literal>&lt;&gt;</> operator. However, if both
inputs are null it returns false, and if only one input is
null it returns true. Similarly, <literal>IS NOT DISTINCT
FROM</literal> is identical to <literal>=</literal> for non-null
inputs, but it returns true when both inputs are null, and false when only
one input is null. Thus, these constructs effectively act as though null
were a normal data value, rather than <quote>unknown</>.
inverse results for row-valued expressions; in particular, a row-valued
expression that contains both null and non-null fields will return false
for both tests. In some cases, it may be preferable to
write <replaceable>row</replaceable> <literal>IS DISTINCT FROM NULL</>
or <replaceable>row</replaceable> <literal>IS NOT DISTINCT FROM NULL</>,
which will simply check whether the overall row value is null without any
additional tests on the row fields.
</para>
<para>