Re: [HACKERS] "op must return bool to be used with subquery"?

Started by Bruce Momjianalmost 27 years ago2 messageshackers
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#1Bruce Momjian
bruce@momjian.us

Given
create table t1 (name text, value float8);

this fails:
SELECT name, value FROM t1 as touter WHERE
(value/(SELECT AVG(value) FROM t1 WHERE name = touter.name)) > 0.75;
ERROR: parser: '/' must return 'bool' to be used with subquery

Aren't you really saying WHERE col / (subselect). That doesn't return
bool, so the message seems correct.

We don't allow subselects in target lists, or inside expressions.

The code in parse_expr.c that produces this message claims to be
enforcing that "sub-selects can only be used in WHERE clauses".
Either the comment is inaccurate or the test is too restrictive.
If the test is correct then I think the error message is unhelpful.
Anybody understand this code well enough to know what's really going on?

regards, tom lane

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#2Tom Lane
tgl@sss.pgh.pa.us
In reply to: Bruce Momjian (#1)

Bruce Momjian <maillist@candle.pha.pa.us> writes:

Given
create table t1 (name text, value float8);

this fails:
SELECT name, value FROM t1 as touter WHERE
(value/(SELECT AVG(value) FROM t1 WHERE name = touter.name)) > 0.75;
ERROR: parser: '/' must return 'bool' to be used with subquery

Aren't you really saying WHERE col / (subselect). That doesn't return
bool, so the message seems correct.

No, look again: the result of the subselect is being used as an operand
within the WHERE clause:
WHERE (value/(SUBSELECT)) > 0.75;

If the / were the toplevel operator in the WHERE then the message would
make sense, because the WHERE clause as a whole must yield boolean.
But that doesn't mean that the operator immediately above the subselect
must yield boolean.

Besides, I already fixed this ;-)

We don't allow subselects in target lists, or inside expressions.

We don't allow 'em in target lists, I think (anyone understand why not?)
but they work fine inside expressions in WHERE or HAVING.

regards, tom lane