BUG #8228: Unexpected "set-valued function" with varchar(n) but not varchar

david.g.johnston@gmail.com writes:

The following query results in "SQL Error: ERROR: set-valued function called
in context that cannot accept a set"

SELECT *, CASE WHEN id = 2 THEN
(regexp_matches(input_string,'^0*([1-9]\d+)$'))[1] ELSE input_string
END::varchar(30) AS o_l2_a
FROM (
VALUES (1,''), (2,'0000000049404'),(3,'FROM 10000000876')
) l0_src (id, input_string)

Hm, interesting example. What seems to be happening is that during
evaluation of the SELECT list for the first VALUES row, the CASE
expression doesn't call regexp_matches() but just returns the ELSE
expression. The ExecMakeFunctionResult() call for the cast function
then decides that the function's argument expression doesn't return a
set, so it changes the node execution pointer so that subsequent
executions go through the much simpler ExecMakeFunctionResultNoSets()
execution function. And then that spits up when on the next row, the
argument expression *does* return a set :-(

You could work around that using the trick documented in the
regexp_matches documentation to force it to return exactly one row,
ie interpose a sub-SELECT:

regression=# SELECT *, CASE WHEN id = 2 THEN
(select (regexp_matches(input_string,'^0*([1-9]\d+)$'))[1]) ELSE input_string
END::varchar(30) AS o_l2_a
FROM (
VALUES (1,''), (2,'0000000049404'),(3,'FROM 10000000876')
) l0_src (id, input_string);
id | input_string | o_l2_a
----+------------------+------------------
1 | |
2 | 0000000049404 | 49404
3 | FROM 10000000876 | FROM 10000000876
(3 rows)

Not sure about non-hack fixes. I guess we need to analyze
can-it-return-a-set statically instead of believing the first execution
result, but that might add an unpleasant amount of startup overhead.

regards, tom lane

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On Thu, Jun 13, 2013 at 4:02 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

david.g.johnston@gmail.com writes:

The following query results in "SQL Error: ERROR: set-valued function

called

in context that cannot accept a set"

SELECT *, CASE WHEN id = 2 THEN
(regexp_matches(input_string,'^0*([1-9]\d+)$'))[1] ELSE input_string
END::varchar(30) AS o_l2_a
FROM (
VALUES (1,''), (2,'0000000049404'),(3,'FROM 10000000876')
) l0_src (id, input_string)

Hm, interesting example. What seems to be happening is that during
evaluation of the SELECT list for the first VALUES row, the CASE
expression doesn't call regexp_matches() but just returns the ELSE
expression.

Does all this explain why it DOES work if the cast on the END is a plain
"varchar"?

Not sure about non-hack fixes. I guess we need to analyze
can-it-return-a-set statically instead of believing the first execution
result, but that might add an unpleasant amount of startup overhead.

regards, tom lane

The issue with the regexp_matches call generally is that absence of a "g"
modifier means that the set-returning function will never return a set. It
would seem to make more sense to not make that a modifier but instead have
one function defined to return a set (i.e., the current definition) and
another one defined to return a simply text[]. This would make using the
call in a scalar context easier. Is there any reason why a UDF defined as
such would have a problem? The set-returning one accepting the parameter
is nice since you can toggle global/single within the same query - but in
many use-cases only the single-match mode is desired.

Are there any other functions that have this same risk profile that would
increase the applicability of such a patch?

David J.

David Johnston <david.g.johnston@gmail.com> writes:

The issue with the regexp_matches call generally is that absence of a "g"
modifier means that the set-returning function will never return a set. It
would seem to make more sense to not make that a modifier but instead have
one function defined to return a set (i.e., the current definition) and
another one defined to return a simply text[].

Well, it does return a set, namely either zero or one row. The point of
the sub-SELECT workaround is to transform the zero-row case to a scalar
NULL.

I tend to agree that this API wasn't that well thought out, but it's
really not regexp_matches()'s fault that you're running into this
problem --- rather, it's the fact that one arm of the CASE can return a
set while the other can't.

regards, tom lane

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