[PATCH] Include triggers in EXPLAIN

Hello!

Recently I got few times into situation where I was trying to find out what
is blocking DELETE queries. Running EXPLAIN (even VERBOSE one) wasn't
useful, since the reason was slow trigger (missing index on foreign key
column). I had to create testing entry and run EXPLAIN ANALYZE DELETE to
get this information.

It will be really valuable for me to show triggers in EXPLAIN query since
it will make clear for me there will be any trigger "activated" during
execution of DELETE query and that can be the reason for slow DELETE.

I have seen initial discussion at
https://www.postgresql.org/message-id/flat/20693.1111732761%40sss.pgh.pa.us
to show time spent in triggers in EXPLAIN ANALYZE including quick
discussion to possibly show triggers during EXPLAIN. Anyway since it
doesn't show any additional cost and just inform about the possibilities, I
still consider this feature useful. This is probably implementation of idea
mentioned at
https://www.postgresql.org/message-id/21221.1111736869%40sss.pgh.pa.us by
Tom Lane.

After initial discussion with Pavel Stěhule and Tomáš Vondra at czech
postgresql maillist (
https://groups.google.com/forum/#!topic/postgresql-cz/Dq1sT7huVho) I was
able to prepare initial version of this patch. I have added EXPLAIN option
called TRIGGERS enabled by default.There's already autoexplain property for
this. I understand it is not possible to show only triggers which will be
really activated unless query is really executed. EXPLAIN ANALYZE remains
untouched with this patch.

- patch with examples can be found at
https://github.com/simi/postgres/pull/2
- DIFF format https://github.com/simi/postgres/pull/2.diff
- PATCH format (also attached) https://github.com/simi/postgres/pull/2.patch

All regression tests passed with this change locally on latest git master.
I would like to cover this patch with more regression tests, but I wasn't
sure where to place them, since there's no "EXPLAIN" related test "group".
Is "src/test/regress/sql/triggers.sql" the best place to add tests related
to this change?

PS: This is my first try to contribute to postgresql codebase. The quality
of patch is probably not the best, but I will be more than happy to do any
requested change if needed.

Regards,
Josef Šimánek