Use an MVCC snapshot, rather than SnapshotNow, for catalog scans.

SnapshotNow scans have the undesirable property that, in the face of
concurrent updates, the scan can fail to see either the old or the new
versions of the row.  In many cases, we work around this by requiring
DDL operations to hold AccessExclusiveLock on the object being
modified; in some cases, the existing locking is inadequate and random
failures occur as a result.  This commit doesn't change anything
related to locking, but will hopefully pave the way to allowing lock
strength reductions in the future.

The major issue has held us back from making this change in the past
is that taking an MVCC snapshot is significantly more expensive than
using a static special snapshot such as SnapshotNow.  However, testing
of various worst-case scenarios reveals that this problem is not
severe except under fairly extreme workloads.  To mitigate those
problems, we avoid retaking the MVCC snapshot for each new scan;
instead, we take a new snapshot only when invalidation messages have
been processed.  The catcache machinery already requires that
invalidation messages be sent before releasing the related heavyweight
lock; else other backends might rely on locally-cached data rather
than scanning the catalog at all.  Thus, making snapshot reuse
dependent on the same guarantees shouldn't break anything that wasn't
already subtly broken.

Patch by me.  Review by Michael Paquier and Andres Freund.
pg_upgrade:  revert changing '' to ""
On the command line, GUC option strings are handled by the guc parser,
not by the shell parser, so '' is the proper way to represent a
zero-length string.  This reverts commit
3132a9b7ab3d76c15f88cfa29792fd888e7a959e.
Silence compiler warning in assertion-enabled builds.

With -Wtype-limits, gcc correctly points out that size_t can never be < 0.
Backpatch to 9.3 and 9.2. It's been like this forever, but in <= 9.1 you got
a lot other warnings with -Wtype-limits anyway (at least with my version of
gcc).

Andres Freund
Mention extra_float_digits in floating point docs

Make it easier for readers of the FP docs to find out about possibly
truncated values.

Per complaint from Tom Duffey in message
F0E0F874-C86F-48D1-AA2A-0C5365BF5118@trillitech.com

Author: Albe Laurenz
Reviewed by: Abhijit Menon-Sen
Add support for multiple kinds of external toast datums.

To that end, support tags rather than lengths for external datums.
As an example of how this can be used, add support or "indirect"
tuples which point to some externally allocated memory containing
a toast tuple.  Similar infrastructure could be used for other
purposes, including, perhaps, support for alternative compression
algorithms.

Andres Freund, reviewed by Hitoshi Harada and myself
pg_restore: Error about incompatible options

This mirrors the equivalent error cases in pg_dump.
doc: Arrange See Also links in more consistent order
Add #include to make header file independent
doc: Remove i18ngurus.com link

The web site is dead, and the Wayback Machine shows that it didn't have
much useful content before.
Expose object name error fields in PL/pgSQL.

Specifically, permit attaching them to the error in RAISE and retrieving
them from a caught error in GET STACKED DIAGNOSTICS.  RAISE enforces
nothing about the content of the fields; for its purposes, they are just
additional string fields.  Consequently, clarify in the protocol and
libpq documentation that the usual relationships between error fields,
like a schema name appearing wherever a table name appears, are not
universal.  This freedom has other applications; consider a FDW
propagating an error from an RDBMS having no schema support.

Back-patch to 9.3, where core support for the error fields was
introduced.  This prevents the confusion of having a release where libpq
exposes the fields and PL/pgSQL does not.

Pavel Stehule, lexical revisions by Noah Misch.
Fix refresh concurrently when unique indexes cover all columns.
Merge branch 'master' into matview
Additional regression tests for CREATE OPERATOR.

Robins Tharakan, reviewed by Szymon Guz
Regression tests for LISTEN/NOTIFY/UNLISTEN/pg_notify.

Robins Tharakan, reviewed by Szymon Guz
Unbreak postmaster restart-after-crash sequence

In patch 82233ce7ea42, AbortStartTime wasn't being reset appropriately
after the restart sequence, causing subsequent iterations through
ServerLoop to malfunction.
Fix handling of auto-updatable views on inherited tables.

An INSERT into such a view should work just like an INSERT into its base
table, ie the insertion should go directly into that table ... not be
duplicated into each child table, as was happening before, per bug #8275
from Rushabh Lathia.  On the other hand, the current behavior for
UPDATE/DELETE seems reasonable: the update/delete traverses the child
tables, or not, depending on whether the view specifies ONLY or not.
Add some regression tests covering this area.

Dean Rasheed
Hopefully-portable regression tests for CREATE/ALTER/DROP COLLATION.

The collate.linux.utf8 test covers some of the same territory, but
isn't portable and so probably does not get run often, or on
non-Linux platforms.  If this approach turns out to be sufficiently
portable, we may want to look at trimming the redundant tests out
of that file to avoid duplication.

Robins Tharakan, reviewed by Michael Paquier and Fabien Coelho,
with further changes and cleanup by me.
Revert "Hopefully-portable regression tests for CREATE/ALTER/DROP COLLATION."

This reverts commit 263645305b8f14a3821e04dffa96fa7c1bc2ae86.

The buildfarm is sad.
pg_buffercache: document column meanings
Improve documentation for usagecount and relforknumber.
Backpatch to 9.3.
Suggestion from Satoshi Nagayasu
Get rid of pg_class.reltoastidxid.

Treat TOAST index just the same as normal one and get the OID
of TOAST index from pg_index but not pg_class.reltoastidxid.
This change allows us to handle multiple TOAST indexes, and
which is required infrastructure for upcoming
REINDEX CONCURRENTLY feature.

Patch by Michael Paquier, reviewed by Andres Freund and me.