Jeff Janes <jeff.janes@gmail.com> writes:
> On Wed, Nov 4, 2015 at 7:14 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> You're missing my point: that is possible in an indexscan, but *not* in a
>> bitmap indexscan, because the index AM APIs are totally different in the
>> two cases. In a bitmap scan, nothing more than a TID bitmap is ever
>> returned out to anyplace that could execute arbitrary expressions.
> I had thought it must already be able to execute arbitrary
> expressions, due to the ability to already support user-defined btree
> ops (and ops of non-btree types in the case of other index types).
No. An index AM is only expected to be able to evaluate clauses of
the form <indexed_column> <indexable_operator> <constant>, and the key
restriction there is that the operator is one that the AM has volunteered
to support. Well, actually, it's the opclass more than the AM that
determines this, but anyway it's not just some random operator; more than
likely, the AM and/or opclass has got special logic about the operator.
This also ties into Robert's point about evaluation of operators against
index entries for dead or invisible rows. Indexable operators are much
less likely than others to have unexpected side-effects.
regards, tom lane