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Remove undocumented IS [NOT] OF syntax.

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This feature was added a long time ago, in 7c1e67bd5 and eb121ba2c,
but never documented in any user-facing way.  (Documentation added
in 6126d3e70 was commented out almost immediately, in 8272fc3f7.)
That's because, while this syntax is defined by SQL:99, our
implementation is only vaguely related to the standard's semantics.
The standard appears to intend a run-time not parse-time test, and
it definitely intends that the test should understand subtype
relationships.

No one has stepped up to fix that in the intervening years, but
people keep coming across the code and asking why it's not documented.
Let's just get rid of it: if anyone ever wants to make it work per
spec, they can easily recover whatever parts of this code are still
of value from our git history.

If there's anyone out there who's actually using this despite its
undocumented status, they can switch to using pg_typeof() instead,
eg. "pg_typeof(something) = 'mytype'::regtype".  That gives
essentially the same semantics as what our IS OF code did.
(We didn't have that function last time this was discussed, or
we would have ripped out IS OF then.)

Discussion: https://postgr.es/m/CAKFQuwZ2pTc-DSkOiTfjauqLYkNREeNZvWmeg12Q-_69D+sYZA@mail.gmail.com
Discussion: https://postgr.es/m/BAY20-F23E9F2B4DAB3E4E88D3623F99B0@phx.gbl
Discussion: https://postgr.es/m/3E7CF81D.1000203@joeconway.com
This commit is contained in:
Tom Lane
2020-11-19 17:39:39 -05:00
parent 97390fe8a6
commit 926fa801ac
12 changed files with 38 additions and 139 deletions
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19
doc/src/sgml/func.sgml
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@ -762,25 +762,6 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
expression must be of Boolean type.
</para>
<!-- IS OF does not conform to the ISO SQL behavior, so it is undocumented here
<para>
<indexterm>
<primary>IS OF</primary>
</indexterm>
<indexterm>
<primary>IS NOT OF</primary>
</indexterm>
It is possible to check the data type of an expression using the
predicates
<synopsis>
<replaceable>expression</replaceable> IS OF (typename, ...)
<replaceable>expression</replaceable> IS NOT OF (typename, ...)
</synopsis>
They return a boolean value based on whether the expression's data
type is one of the listed data types.
</para>
-->
<para>
Some comparison-related functions are also available, as shown in <xref
linkend="functions-comparison-func-table"/>.