On Mon, Dec 12, 2016 at 10:08 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:

> Every WHERE clause is a
boolean expression, so there's no principled reason why such a rule
> wouldn't result in canonicalizing e.g. "i = 42" into "(i = 42) = true",
> wreaking havoc on every other optimization we have.  Restricting it
> to only apply to simple boolean columns is no answer because (a) indexes
> can be on boolean expressions not just simple columns, and (b) part
> of the point of the canonicalization is to make logically-equivalent
> expressions look alike, so declining to apply it in some cases would
> ruin that.

​Given our reliance on operators in indexing a canonicalization ​rule that
says all boolean yielding expressions must contain an operator would yield
the desired result.  "i = 42" already has an operator so no further
normalization is necessary to make it conform to such a rule.

The indexing concern may still come into play here, I'm not familiar enough
with indexes on column lists versus indexes on expressions to know off the
top of my head.  The definition of "looking alike" seems like it would be
met since all such expression would look alike in having an operator.

That said, not adding "= true" is more visually appealing - though given
all of the other things we do in ruleutils this seems like a minor addition.

David J.

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