RE: Skipping NULL keys when uniqueifying a semijoin's RHS

Hi Ilmari,

Thanks for the review - v2 attached.

> [use foreach_node for the loop over join_info_list]

Done.

> [and for the loop over semi_rhs_exprs]

This one I couldn't take as written, so I used foreach_ptr
instead.

Two further changes:

First, I dropped this guard:

/* Nothing reads the whole RHS unless it can be unique-ified */
if (!sjinfo->semi_can_btree && !sjinfo->semi_can_hash)
continue;

The check was unreachable. compute_semijoin_info() assigns
sjinfo->semi_rhs_exprs only after it has already returned early on
!(all_btree || all_hash), so semi_rhs_exprs is NIL whenever both flags
are false.

Second, I added a strictness check:

/* A non-strict operator can match a NULL key */
if (!op_strict(opno))
continue;

compute_semijoin_info() requires each operator to be hashjoinable or
mergejoinable, but nothing requires it to be strict.

Thanks,
Will