Re: [PATCH] Caching for stable expressions with constant arguments v3

On Mon, Dec 5, 2011 at 19:31, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I think if you have some call sites inject CacheExprs and others not,
> it will get more difficult to match up expressions that should be
> considered equal.  On the whole this seems like a bad idea.  What is
> the reason for having such a control boolean in the first place?

It's needed for correctness with PL/pgSQL simple expressions.

This seems a bit of a kludge, but I considered it the "least bad"
solution. Here's what I added to planner.c standard_planner():

/** glob->isSimple is a hint to eval_const_expressions() and PL/pgSQL that* this statement is potentially a simple
expression-- it contains no* table references, no subqueries and no join clauses.** We need this here because this
preventsinsertion of CacheExpr, which* would break simple expressions in PL/pgSQL. Such queries wouldn't* benefit from
constantcaching anyway.** The actual definition of a simple statement is more strict, but we* don't want to spend that
checkingoverhead here.** Caveat: Queries with set-returning functions in SELECT list could* still potentially benefit
fromcaching, but we don't check that now.*/ 
glob->isSimple = (parse->commandType == CMD_SELECT &&              parse->jointree->fromlist == NIL &&
parse->hasSubLinks== FALSE &&              parse->cteList == NIL); 

----

I considered stripping CacheExpr nodes later in PL/pgSQL, but I can't
remember right now why I rejected that approach (sorry, it's been 2
months).

Currently I'm also disabling CacheExpr nodes in
estimate_expression_value() since we know for a fact that the planner
only evaluates it once. But that probably doesn't make much of a
difference.

Regards,
Marti