Use LEFT JOINs in some system views in case referenced row doesn't exist.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 8299471c37fff0b0f5a777a12f920125310c0efe
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2016-08-19T21:13:47Z
Releases: 10.0
Use LEFT JOINs in some system views in case referenced row doesn't exist.

In particular, left join to pg_authid so that rows in pg_stat_activity
don't disappear if the session's owning user has been dropped.
Also convert a few joins to pg_database to left joins, in the same spirit,
though that case might be harder to hit.  We were doing this in other
views already, so it was a bit inconsistent that these views didn't.

Oskari Saarenmaa, with some further tweaking by me

Discussion: <56E87CD8.60007@ohmu.fi>

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