How embarrassing: optimization of a one-shot query doesn't work

While testing the changes I was making to Pavel's EXECUTE USING patch
to ensure that parameter values were being provided to the planner,
it became painfully obvious that the planner wasn't actually *doing*
anything with them. For example

execute 'select count(*) from foo where x like $1' into c using $1;

wouldn't generate an indexscan when $1 was of the form 'prefix%'.

Some investigation showed that the planner is using the passed values
for estimation purposes, but not for any purposes where the value *must*
be correct (not only this LIKE-optimization, but constraint exclusion,
for instance). The reason is that the parameter values are made
available to estimate_expression_value but not to eval_const_expressions.
This is a thinko in a cleanup patch I made early in 8.3 development:
http://archives.postgresql.org/pgsql-committers/2007-02/msg00352.php
I said to myself "eval_const_expressions doesn't need any context,
because a constant expression's value must be independent of context,
so I can avoid changing its API". Silly me.

The implication of this is that 8.3 is significantly worse than 8.2
in optimizing unnamed statements in the extended-Query protocol;
a feature that JDBC, at least, relies on.

The fix is simple: add PlannerInfo to eval_const_expressions's
parameter list, as was done for estimate_expression_value. I am
slightly hesitant to do this in a stable branch, since it would break
any third-party code that might be calling that function. I doubt there
is currently any production-grade code doing so, but if anyone out there
is actively using those planner hooks we put into 8.3, it's conceivable
this would affect them.

Still, the performance regression here is bad enough that I think there
is little choice. Comments/objections?

regards, tom lane