Re: Patch for 8.5, transformationHook

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> 2009/4/11 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>> No, I was complaining that a hook right there is useless and expensive.
>> transformExpr() is executed multiple times per query, potentially a very
>> large number of times per query; so even testing to see if a hook exists
>> is not a negligible cost.

> I did some tests based on pgbench.

The queries done by pgbench are completely trivial and do not stress
parser performance. Even if they did (consider cases likw an IN with a
few thousand list items), the parser is normally not a bottleneck
compared to transaction overhead, network round trips, and pgbench
itself.

> I though about different position of hook, but only in this place the
> hook is useful (because expressions are recursive).

As I keep saying, a hook there is useless, at least by itself. You
have no control over the grammar and no ability to modify what the
rest of the system understands. The only application I can think of is
to fool with the transformation of FuncCall nodes, which you could do in
a much lower-overhead way by hooking into transformFuncCall. Even that
seems pretty darn marginal for real-world problems.

regards, tom lane