From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Daniel Gustafsson <daniel(at)yesql(dot)se> |
Cc: | aceonline(at)gmx(dot)de, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
Subject: | Re: BUG #16510: Count Distinct with non distinct column in combination with string constants throws error |
Date: | 2020-06-25 14:32:34 |
Message-ID: | 2270140.1593095554@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
Daniel Gustafsson <daniel(at)yesql(dot)se> writes:
> PostgreSQL doesn't know which datatype you expect 'blub' to be, as it isn't
> related to the testtable relation in your query. If you cast to the datatype
> of your choice you will get the expected result.
> postgres=# SELECT COUNT(DISTINCT(testtable.column2, 'blub')) FROM public.testtable;
> ERROR: could not identify a comparison function for type unknown
The reason for this might be a little more obvious if you wrote the
implicit row constructor explicitly, ie
SELECT COUNT(DISTINCT ROW(testtable.column2, 'blub')) FROM public.testtable;
The row's datatype is indeterminate as-specified.
Perhaps there's room to argue that we should allow 'unknown' to decay to
'text' automatically in this context, but I'm not in a big hurry to do
that. It seems better to make people be explicit about which datatype
they intend inside such complex, infrequently-used constructs.
regards, tom lane
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