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Doc: fix description of UNION/CASE/etc type unification. 

The description of what select_common_type() does was not terribly
accurate.  Improve it.

David Johnston and Tom Lane

Discussion: https://postgr.es/m/1019930.1597613200@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2020-08-17 15:40:07 -04:00
parent a28d731a11
commit 22e75a341e
1 changed files with 19 additions and 14 deletions
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33
doc/src/sgml/typeconv.sgml
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@ -1069,7 +1069,7 @@ domain's base type for all subsequent steps.
functions, this behavior allows a domain type to be preserved through
a <literal>UNION</literal> or similar construct, so long as the user is
careful to ensure that all inputs are implicitly or explicitly of that
exact type. Otherwise the domain's base type will be preferred.
exact type. Otherwise the domain's base type will be used.
</para>
</footnote>
</para>
@ -1092,24 +1092,29 @@ If the non-unknown inputs are not all of the same type category, fail.
<step performance="required">
<para>
Choose the first non-unknown input type which is a preferred type in
that category, if there is one.
Select the first non-unknown input type as the candidate type,
then consider each other non-unknown input type, left to right.
<footnote>
<para>
For historical reasons, <literal>CASE</literal> treats
its <literal>ELSE</literal> clause (if any) as the <quote>first</quote>
input, with the <literal>THEN</literal> clauses(s) considered after
that. In all other cases, <quote>left to right</quote> means the order
in which the expressions appear in the query text.
</para>
</footnote>
If the candidate type can be implicitly converted to the other type,
but not vice-versa, select the other type as the new candidate type.
Then continue considering the remaining inputs. If, at any stage of this
process, a preferred type is selected, stop considering additional
inputs.
</para>
</step>
<step performance="required">
<para>
Otherwise, choose the last non-unknown input type that allows all the
preceding non-unknown inputs to be implicitly converted to it. (There
always is such a type, since at least the first type in the list must
satisfy this condition.)
</para>
</step>
<step performance="required">
<para>
Convert all inputs to the selected type. Fail if there is not a
conversion from a given input to the selected type.
Convert all inputs to the final candidate type. Fail if there is not an
implicit conversion from a given input type to the candidate type.
</para>
</step>
</procedure>