Re: contraints_exclusion fails to refute simple condition

From: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: contraints_exclusion fails to refute simple condition
Date: 2015-03-05 16:51:47
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>>>>> "Sandro" == Sandro Santilli <strk(at)keybit(dot)net> writes:

Sandro> PostGIS installs standard constraints of this kind:

Sandro> CHECK (geometrytype(g) = 'POINT'::text OR g IS NULL)

If geometrytype() is strict, the IS NULL condition is superfluous.
CHECK(x) _passes_ when x is null, rather than rejecting the row.

Sandro> But it is _NOT_ used if the NOT NULL condition is removed:

Sandro> WHERE geometrytype(g) = 'LINESTRING'

Sandro> As the "geometrytype" is defined as STRICT and IMMUTABLE,
Sandro> there's no way for geometrytype(g) = 'LINESTRING' to hold true,
Sandro> so why is the "IS NOT NULL" condition also needed by the
Sandro> planner ?

Sandro> Andres Freund on IRC suggested that
Sandro> predicate_refuted_by_simple_clause() looks like trying to
Sandro> handle such cases, but if that's the case it seems to fail
Sandro> here.

It looks like it searches only one level deep into the clause, so it
knows that if OP or FUNC is strict that (g OP const) or (FUNC(g)) will
refute (g IS NULL), but not that (FUNC(g) OP const) will do so, because
the reference to g is nested too deep.

In this case the obvious thing to do is remove the redundant clause.
Whether it would be worth teaching the predicate prover to recurse in
such cases would be a performance tradeoff - likely a poor one, since it
would have to recurse into arbitrarily complex expressions.

Andrew (irc:RhodiumToad)

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