Re: Report error position in partition bound check

I looked this over and pushed it with some minor adjustments.

However, while I was looking at it I couldn't help noticing that
transformPartitionBoundValue's handling of collation concerns seems
less than sane. There are two things bugging me:

1. Why does it care about the expression's collation only when there's
a top-level CollateExpr? For example, that means we get an error for

regression=# create table p (f1 text collate "C") partition by list(f1);
CREATE TABLE
regression=# create table c1 partition of p for values in ('a' collate "POSIX");
ERROR: collation of partition bound value for column "f1" does not match partition key collation "C"

but not this:

regression=# create table c2 partition of p for values in ('a' || 'b' collate "POSIX");
CREATE TABLE

Given that we will override the expression's collation with the partition
column's collation anyway, I don't see why we have this check at all,
so my preference is to just rip out the entire stanza beginning with
"if (IsA(value, CollateExpr))". If we keep it, though, I think it needs
to do something else that is more general.

2. Nothing is doing assign_expr_collations() on the partition expression.
This can trivially be shown to cause problems:

regression=# create table p (f1 bool) partition by list(f1);
CREATE TABLE
regression=# create table cf partition of p for values in ('a' < 'b');
ERROR: could not determine which collation to use for string comparison
HINT: Use the COLLATE clause to set the collation explicitly.

If we want to rip out the collation mismatch error altogether, then
fixing #2 would just require inserting assign_expr_collations() before
the expression_planner() call. The other direction that would make
sense to me is to perform assign_expr_collations() after
coerce_to_target_type(), and then to complain if exprCollation()
isn't default and doesn't match the partition collation. In any
case a specific test for a CollateExpr seems quite wrong.

regards, tom lane