The following bug has been logged on the website:
Bug reference: 14515
Logged by: Tom Dunstan
Email address: pgsql@tomd.cc
PostgreSQL version: 9.5.5
Operating system: Linux
Description:
Hi all
Not sure if this is a bug or just ambiguity in the docs, but I certainly
found it counter-intuitive.
Basically while `select 'somethingelse'::tsvector @@ '!foo'::tsquery`
returns true, `select ''::tsvector @@ '!foo'::tsquery` returns false, which
was surprising to me.
I would expect a tsvector (empty or not) that doesn't contain foo to match
!foo.
It seems like the behaviour is "match a term that isn't foo" rather than
"match if no terms match foo".
Not sure if this is intentional (in which case maybe the docs could be a bit
more clear) or if it's just a bug. IMO the former behaviour isn't very
useful as it forces us to either code around it by writing extra SQL to
handle the case, or insert a dummy term to match against. Neither is very
nice.
It may be unintentional: there's a fast-path out of the match function here
https://github.com/postgres/postgres/blob/f21a563d25dbae153937aec062161184189478b8/src/backend/utils/adt/tsvector_op.c#L1930-L1935
that returns false when the vector is empty. Perhaps it just wasn't taking
the negative-only case into account.
Cheers
Tom
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