The authors of bbe0a81d hadn't quite got the idea that macros named
like SOMETHING_4B_C were only meant for internal endianness-related
details in postgres.h.  Choose more legible names for macros that are
intended to be used elsewhere.  Rearrange postgres.h a bit to clarify
the separation between those internal macros and ones intended for
wider use.

Also, avoid using the term "rawsize" for true decompressed size;
we've used "extsize" for that, because "rawsize" generally denotes
total Datum size including header.  This choice seemed particularly
unfortunate in tests that were comparing one of these meanings to
the other.

This patch includes a couple of not-purely-cosmetic changes: be
sure that the shifts aligning compression methods are unsigned
(not critical today, but will be when compression method 2 exists),
and fix broken definition of VARATT_EXTERNAL_GET_COMPRESSION (now
VARATT_EXTERNAL_GET_COMPRESS_METHOD), whose callers worked only
accidentally.

Discussion: https://postgr.es/m/574197.1616428079@sss.pgh.pa.us

This seems to have been just copied-and-pasted from some other
header checks.  But our C code is entirely unprepared to support
such a header name, so it's only wasting cycles to look for it.
If we did need to support it, some #ifdefs would be required.

(A quick trawl at codesearch.debian.net finds some packages that
reference lz4/lz4.h; but they use *only* that spelling, and
appear to be intending to reference their own copy rather than
a system-level installation of liblz4.  There's no evidence of
freestanding installations that require this spelling.)

Discussion: https://postgr.es/m/457962.1616362509@sss.pgh.pa.us

The previous coding treated an invalid compression method name as
equivalent to the default, which is certainly not right.

Justin Pryzby

Discussion: http://postgr.es/m/20210321235544.GD4203@telsasoft.com

Previously, the default_toast_compression GUC was not documented,
and neither was pg_dump's new --no-toast-compression option.

Justin Pryzby and Robert Haas

Discussion: http://postgr.es/m/20210321235544.GD4203@telsasoft.com

Remove unused macro. Fix confusion about whether a TOAST compression
method is identified by an OID or a char.

Justin Pryzby

Discussion: http://postgr.es/m/20210321235544.GD4203@telsasoft.com

This commit is mostly a revert of aaa3aedd, that switched the routine
doing the internal renaming of recycled WAL segments to use on Windows a
combination of CreateHardLinkA() plus unlink() instead of rename().  As
reported by several users of Postgres 13, this is causing concurrency
issues when manipulating WAL segments, mostly in the shape of the
following error:
LOG:  could not rename file "pg_wal/000000XX000000YY000000ZZ":
Permission denied

This moves back to a logic where a single rename() (well, pgrename() for
Windows) is used.  This issue has proved to be hard to hit when I tested
it, facing it only once with an archive_command that was not able to do
its work, so it is environment-sensitive.  The reporters of this issue
have been able to confirm that the situation improved once we switched
back to a single rename().  In order to check things, I have provided to
the reporters a patched build based on 13.2 with aaa3aedd reverted, to
test if the error goes away, and an unpatched build of 13.2 to test if
the error still showed up (just to make sure that I did not mess up my
build process).

Extra thanks to Fujii Masao for pointing out what looked like the
culprit commit, and to all the reporters for taking the time to test
what I have sent them.

Reported-by: Andrus, Guy Burgess, Yaroslav Pashinsky, Thomas Trenz
Reviewed-by: Tom Lane, Andres Freund
Discussion: https://postgr.es/m/3861ff1e-0923-7838-e826-094cc9bef737@hot.ee
Discussion: https://postgr.es/m/16874-c3eecd319e36a2bf@postgresql.org
Discussion: https://postgr.es/m/095ccf8d-7f58-d928-427c-b17ace23cae6@burgess.co.nz
Discussion: https://postgr.es/m/16927-67c570d968c99567%40postgresql.org
Discussion: https://postgr.es/m/YFBcRbnBiPdGZvfW@paquier.xyz
Backpatch-through: 13

This commit improves pgbench \sleep command so that it handles
the following three cases more properly.

(1) When only one argument was specified in \sleep command and
    it's not a number, previously pgbench reported a confusing error
    message like "unrecognized time unit, must be us, ms or s".
    This commit fixes this so that more proper error message like
    "invalid sleep time, must be an integer" is reported.

(2) When two arguments were specified in \sleep command and
    the first argument was not a number, previously pgbench treated
    that argument as the sleep time 0. No error was reported in this
    case. This commit fixes this so that an error is thrown in this
    case.

(3) When a variable was specified as the first argument in \sleep
    command and the variable stored non-digit value, previously
    pgbench treated that argument as the sleep time 0. No error
    was reported in this case. This commit fixes this so that
    an error is thrown in this case.

Author: Kota Miyake
Reviewed-by: Hayato Kuroda, Alvaro Herrera, Fujii Masao
Discussion: https://postgr.es/m/23b254daf20cec4332a2d9168505dbc9@oss.nttdata.com

Back-patch to v13, which introduced the test code in question.

The test added by 595b9cba forgot that on Windows it is necessary to set
up pg_hba.conf (see PostgresNode::set_replication_conf) with a specific
entry or base backups fail.  Any node that requires to support
replication just needs to pass down allows_streaming at initialization.
This updates the test to do so.  Simplify things a bit while on it.

Per buildfarm member fairywren.  Any Windows hosts running this test
would have failed, and I have reproduced the problem as well.

Backpatch-through: 10

The TAP tests of kerberos rely on the logs generated by the backend to
check various connection scenarios.  In order to make sure that a given
test does not overlap with the log contents generated by a previous
test, the test suite relied on a logic with the logging collector and a
rotation of the log files to ensure the uniqueness of the log generated
with a wait phase.

Parsing the log contents for expected patterns is a problem that has
been solved in a simpler way by PostgresNode::issues_sql_like() where
the log file is truncated before checking for the contents generated,
with the backend sending its output to a log file given by pg_ctl
instead.  This commit switches the kerberos test suite to use such a
method, removing any wait phase and simplifying the whole logic,
resulting in less code.  If a failure happens in the tests, the contents
of the logs are still showed to the user at the moment of the failure
thanks to like(), so this has no impact on debugging capabilities.

I have bumped into this issue while reviewing a different patch set
aiming at extending the kerberos test suite to check for multiple
log patterns instead of one now.

Author: Michael Paquier
Reviewed-by: Stephen Frost, Bharath Rupireddy
Discussion: https://postgr.es/m/YFXcq2vBTDGQVBNC@paquier.xyz

Any transactions found as still prepared by a checkpoint have their
state data read from the WAL records generated by PREPARE TRANSACTION
before being moved into their new location within pg_twophase/.  While
reading such records, the WAL reader uses the callback
read_local_xlog_page() to read a page, that is shared across various
parts of the system.  This callback, since 1148e22a, has introduced an
update of ThisTimeLineID when reading a record while in recovery, which
is potentially helpful in the context of cascading WAL senders.

This update of ThisTimeLineID interacts badly with the checkpointer if a
promotion happens while some 2PC data is read from its record, as, by
changing ThisTimeLineID, any follow-up WAL records would be written to
an timeline older than the promoted one.  This results in consistency
issues.  For instance, a subsequent server restart would cause a failure
in finding a valid checkpoint record, resulting in a PANIC, for
instance.

This commit changes the code reading the 2PC data to reset the timeline
once the 2PC record has been read, to prevent messing up with the static
state of the checkpointer.  It would be tempting to do the same thing
directly in read_local_xlog_page().  However, based on the discussion
that has led to 1148e22a, users may rely on the updates of
ThisTimeLineID when a WAL record page is read in recovery, so changing
this callback could break some cases that are working currently.

A TAP test reproducing the issue is added, relying on a PITR to
precisely trigger a promotion with a prepared transaction still
tracked.

Per discussion with Heikki Linnakangas, Kyotaro Horiguchi, Fujii Masao
and myself.

Author: Soumyadeep Chakraborty, Jimmy Yih, Kevin Yeap
Discussion: https://postgr.es/m/CAE-ML+_EjH_fzfq1F3RJ1=XaaNG=-Jz-i3JqkNhXiLAsM3z-Ew@mail.gmail.com
Backpatch-through: 10

It's not okay to scribble directly on a syscache entry.
Nor to continue accessing said entry after releasing it.

Also get rid of not-used local variables.

Per valgrind testing.

Maintain a simple array of metadata about pages that were deleted during
nbtree VACUUM's current btvacuumscan() call.  Use this metadata at the
end of btvacuumscan() to attempt to place newly deleted pages in the FSM
without further delay.  It might not yet be safe to place any of the
pages in the FSM by then (they may not be deemed recyclable), but we
have little to lose and plenty to gain by trying.  In practice there is
a very good chance that this will work out when vacuuming larger
indexes, where scanning the index naturally takes quite a while.

This commit doesn't change the page recycling invariants; it merely
improves the efficiency of page recycling within the confines of the
existing design.  Recycle safety is a part of nbtree's implementation of
what Lanin & Shasha call "the drain technique".  The design happens to
use transaction IDs (they're stored in deleted pages), but that in
itself doesn't align the cutoff for recycle safety to any of the
XID-based cutoffs used by VACUUM (e.g., OldestXmin).  All that matters
is whether or not _other_ backends might be able to observe various
inconsistencies in the tree structure (that they cannot just detect and
recover from by moving right).  Recycle safety is purely a question of
maintaining the consistency (or the apparent consistency) of a physical
data structure.

Note that running a simple serial test case involving a large range
DELETE followed by a VACUUM VERBOSE will probably show that any newly
deleted nbtree pages are not yet reusable/recyclable.  This is expected
in the absence of even one concurrent XID assignment.  It is an old
implementation restriction.  In practice it's unlikely to be the thing
that makes recycling remain unsafe, at least with larger indexes, where
recycling newly deleted pages during the same VACUUM actually matters.

An important high-level goal of this commit (as well as related recent
commits e5d8a999 and 9f3665fb) is to make expensive deferred cleanup
operations in index AMs rare in general.  If index vacuuming frequently
depends on the next VACUUM operation finishing off work that the current
operation started, then the general behavior of index vacuuming is hard
to predict.  This is relevant to ongoing work that adds a vacuumlazy.c
mechanism to skip index vacuuming in certain cases.  Anything that makes
the real world behavior of index vacuuming simpler and more linear will
also make top-down modeling in vacuumlazy.c more robust.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wzk76_P=67iUscb1UN44-gyZL-KgpsXbSxq_bdcMa7Q+wQ@mail.gmail.com

It's not okay to just shove the pkg_config results right into our
build flags, for a couple different reasons:

* This fails to maintain the separation between CPPFLAGS and CFLAGS,
as well as that between LDFLAGS and LIBS.  (The CPPFLAGS angle is,
I believe, the reason for warning messages reported when building
with MacPorts' liblz4.)

* If pkg_config emits anything other than -I/-D/-L/-l switches,
it's highly unlikely that we want to absorb those.  That'd be more
likely to break the build than do anything helpful.  (Even the -D
case is questionable; but we're doing that for libxml2, so I kept it.)

Also, it's not okay to skip doing an AC_CHECK_LIB probe, as
evidenced by recent build failure on topminnow; that should
have been caught at configure time.

Model fixes for this on configure's libxml2 support.

It appears that somebody overlooked an autoheader run, too.

Discussion: https://postgr.es/m/20210119190720.GL8560@telsasoft.com

This test will fail in "make installcheck" if the installation's
default_toast_compression setting is not 'pglz'.  Make it robust
against that situation.

Dilip Kumar

Discussion: https://postgr.es/m/CAFiTN-t0w+Rc2U3S+y=7KWcLuOYNB5MfWeGdNa7+pg0UovVdcQ@mail.gmail.com

This reverts commit 677271a3.
"Unbreak recovery test on Windows"

The test hangs on Windows, and attempts to remedy the problem have
proved fragile at best. So we simply disable the test on Windows perl.
(Msys perl seems perfectly happy).

Discussion: https://postgr.es/m/5b748470-7335-5439-e876-6a88c951e1c5@dunslane.net

My oversight in commit 9aa491ab.

Per coverity.

On Windows we need to send explicit quit messages to psql or the TAP tests
can hang.

Compilers that don't understand that elog(ERROR) doesn't return
issued warnings here.  In the cases in libpq_pipeline.c, we were
not exactly helping things by failing to mark pg_fatal() as noreturn.

Per buildfarm.

The documentation specifically states that lwlock-release fires before
any released waiters have been awakened.  It worked that way until
ab5194e6, where is seems to have been
misplaced accidentally.  Move it back where it belongs.

Author: Craig Ringer <craig.ringer@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAGRY4nwxKUS_RvXFW-ugrZBYxPFFM5kjwKT5O+0+Stuga5b4+Q@mail.gmail.com

When compressing the BRIN summary, we can't simply use the compression
method from the indexed attribute.  The summary may use a different data
type, e.g. fixed-length attribute may have varlena summary, leading to
compression failures.  For the built-in BRIN opclasses this happens to
work, because the summary uses the same data type as the attribute.

When the data types match, we can inherit use the compression method
specified for the attribute (it's copied into the index descriptor).
Otherwise we don't have much choice and have to use the default one.

Author: Tomas Vondra
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/e0367f27-392c-321a-7411-a58e1a7e4817%40enterprisedb.com

The approach used in commit bbe0a81d would've been disastrous for
portability of dumps.  Instead handle non-default compression options
in separate ALTER TABLE commands.  This reduces chatter for the
common case where most columns are compressed the same way, and it
makes it possible to restore the dump to a server that lacks any
knowledge of per-attribute compression options (so long as you're
willing to ignore syntax errors from the ALTER TABLE commands).

There's a whole lot left to do to mop up after bbe0a81d, but
I'm fast-tracking this part because we need to see if it's
enough to make the buildfarm's cross-version-upgrade tests happy.

Justin Pryzby and Tom Lane

Discussion: https://postgr.es/m/20210119190720.GL8560@telsasoft.com

While back-patching e0e569e1, I noted that there were some other
places where we ought to be applying DH_free(); namely, where we
load some DH parameters from a file and then reject them as not
being sufficiently secure.  While it seems really unlikely that
anybody would hit these code paths in production, let alone do
so repeatedly, let's fix it for consistency.

Back-patch to v10 where this code was introduced.

Discussion: https://postgr.es/m/16160-18367e56e9a28264@postgresql.org

RestoreGUCState applied InitializeOneGUCOption to already-live
GUC entries, causing any malloc'd subsidiary data to be forgotten.
We do want the effect of resetting the GUC to its compiled-in
default, and InitializeOneGUCOption seems like the best way to do
that, so add code to free any existing subsidiary data beforehand.

The interaction between can_skip_gucvar, SerializeGUCState, and
RestoreGUCState is way more subtle than their opaque comments
would suggest to an unwary reader.  Rewrite and enlarge the
comments to try to make it clearer what's happening.

Remove a long-obsolete assertion in read_nondefault_variables: the
behavior of set_config_option hasn't depended on IsInitProcessingMode
since f5d9698a installed a better way of controlling it.

Although this is fixing a clear memory leak, the leak is quite unlikely
to involve any large amount of data, and it can only happen once in the
lifetime of a worker process.  So it seems unnecessary to take any
risk of back-patching.

Discussion: https://postgr.es/m/4105247.1616174862@sss.pgh.pa.us

Since commit 2ce439f3 we have opened every file in the data directory
and called fsync() at the start of crash recovery.  This can be very
slow if there are many files, leading to field complaints of systems
taking minutes or even hours to begin crash recovery.

Provide an alternative method, for Linux only, where we call syncfs() on
every possibly different filesystem under the data directory.  This is
equivalent, but avoids faulting in potentially many inodes from
potentially slow storage.

The new mode comes with some caveats, described in the documentation, so
the default value for the new setting is "fsync", preserving the older
behavior.
Reported-by: Michael Brown <michael.brown@discourse.org>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Paul Guo <guopa@vmware.com>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: David Steele <david@pgmasters.net>
Discussion: https://postgr.es/m/11bc2bb7-ecb5-3ad0-b39f-df632734cd81%40discourse.org
Discussion: https://postgr.es/m/CAEET0ZHGnbXmi8yF3ywsDZvb3m9CbdsGZgfTXscQ6agcbzcZAw%40mail.gmail.com

The function added in be45be9c is comparing integer lists (IntList) by
length and contents, but there were two bugs.  Firstly, it used intVal()
to extract the value, but that's for Value nodes, not for extracting int
values from IntList.  Secondly, it called it directly on the ListCell,
without doing lfirst().  So just do lfirst_int() instead.

Interestingly enough, this did not cause any crashes on the buildfarm,
but valgrind rightfully complained about it.

Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org

Introduced by commit bbe0a81d.

Per buildfarm member prion.

There is now a per-column COMPRESSION option which can be set to pglz
(the default, and the only option in up until now) or lz4. Or, if you
like, you can set the new default_toast_compression GUC to lz4, and
then that will be the default for new table columns for which no value
is specified. We don't have lz4 support in the PostgreSQL code, so
to use lz4 compression, PostgreSQL must be built --with-lz4.

In general, TOAST compression means compression of individual column
values, not the whole tuple, and those values can either be compressed
inline within the tuple or compressed and then stored externally in
the TOAST table, so those properties also apply to this feature.

Prior to this commit, a TOAST pointer has two unused bits as part of
the va_extsize field, and a compessed datum has two unused bits as
part of the va_rawsize field. These bits are unused because the length
of a varlena is limited to 1GB; we now use them to indicate the
compression type that was used. This means we only have bit space for
2 more built-in compresison types, but we could work around that
problem, if necessary, by introducing a new vartag_external value for
any further types we end up wanting to add. Hopefully, it won't be
too important to offer a wide selection of algorithms here, since
each one we add not only takes more coding but also adds a build
dependency for every packager. Nevertheless, it seems worth doing
at least this much, because LZ4 gets better compression than PGLZ
with less CPU usage.

It's possible for LZ4-compressed datums to leak into composite type
values stored on disk, just as it is for PGLZ. It's also possible for
LZ4-compressed attributes to be copied into a different table via SQL
commands such as CREATE TABLE AS or INSERT .. SELECT.  It would be
expensive to force such values to be decompressed, so PostgreSQL has
never done so. For the same reasons, we also don't force recompression
of already-compressed values even if the target table prefers a
different compression method than was used for the source data.  These
architectural decisions are perhaps arguable but revisiting them is
well beyond the scope of what seemed possible to do as part of this
project.  However, it's relatively cheap to recompress as part of
VACUUM FULL or CLUSTER, so this commit adjusts those commands to do
so, if the configured compression method of the table happens not to
match what was used for some column value stored therein.

Dilip Kumar. The original patches on which this work was based were
written by Ildus Kurbangaliev, and those were patches were based on
even earlier work by Nikita Glukhov, but the design has since changed
very substantially, since allow a potentially large number of
compression methods that could be added and dropped on a running
system proved too problematic given some of the architectural issues
mentioned above; the choice of which specific compression method to
add first is now different; and a lot of the code has been heavily
refactored.  More recently, Justin Przyby helped quite a bit with
testing and reviewing and this version also includes some code
contributions from him. Other design input and review from Tomas
Vondra, Álvaro Herrera, Andres Freund, Oleg Bartunov, Alexander
Korotkov, and me.

Discussion: http://postgr.es/m/20170907194236.4cefce96%40wp.localdomain
Discussion: http://postgr.es/m/CAFiTN-uUpX3ck%3DK0mLEk-G_kUQY%3DSNOTeqdaNRR9FMdQrHKebw%40mail.gmail.com

The TAP test was written so that it was not waiting for the correct SQL
command, but for output from the preceding one.  This resulted in race
conditions, allowing the commands to run in a different order, not block
as expected and so on.  This fixes it by inverting the order of commands
where possible, so that observing the output guarantees the data was
inserted properly, and waiting for a lock to appear in pg_locks.

Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

Buildfarm member fairywren doesn't like the test case I added
in commit 081876d7.  I'm guessing the reason is that I shouldn't
be using a perl2host-ified path in the tar command line.

Commit 86c23a6e changed the option to specify that postgres will
stop all other server processes by sending the signal SIGSTOP,
from -s to -T. But previously there were comments incorrectly
explaining that SIGSTOP behavior is set by -s option. This commit
fixes them.

Author: Kyotaro Horiguchi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/20210316.165141.1400441966284654043.horikyota.ntt@gmail.com

We leaked the error report from PQconninfoParse, when there was
one.  It seems unlikely that real usage patterns would repeat
the failure often enough to create serious bloat, but let's
back-patch anyway to keep the code similar in all branches.

Found via valgrind testing.
Back-patch to v10 where this code was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Because guc.c prefers to keep all its string values in malloc'd
not palloc'd storage, it has to be more careful than usual to
avoid leaks.  Error exits out of string GUC hook checks failed
to clear the proposed value string, and error exits out of
ProcessGUCArray() failed to clear the malloc'd results of
ParseLongOption().

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

The text search cache mechanisms assume that we can clean up
an invalidated dictionary cache entry simply by resetting the
associated long-lived memory context.  However, that does not work
for ispell affixes that make use of regular expressions, because
the regex library deals in plain old malloc.  Hence, we leaked
compiled regex(es) any time we dropped such a cache entry.  That
could quickly add up, since even a fairly trivial regex can use up
tens of kB, and a large one can eat megabytes.  Add a memory context
callback to ensure that a regex gets freed when its owning cache
entry is cleared.

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Some code paths in this function perform syscache lookups, which
can lead to table accesses and possibly leakage of cruft into
the caller's context.  If said context is CacheMemoryContext,
we eventually will have visible bloat.  But fixing this is no
harder than moving one memory context switch step.  (The other
callers don't have a problem.)

Andres Freund and I independently found this via valgrind testing.
Back-patch to v12 where this code was added.

Discussion: https://postgr.es/m/20210317023101.anvejcfotwka6gaa@alap3.anarazel.de
Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Although these lists are usually NIL, and even when not empty
are unlikely to be large, constant relcache update traffic could
eventually result in visible bloat of CacheMemoryContext.

Found via valgrind testing.
Back-patch to v10 where this field was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

The test included in cd91de0d had two simple flaws.

Firstly, the number of rows was low and on some platforms (e.g. 32-bit)
the sort did not require on-disk sort, so on those machines it was not
testing the automatic removal.  The test was however failing, because
without any temporary files the base/pgsql_tmp directory was not even
created.  Fixed by increasing the rowcount to 5000, which should be high
engough on any platform.

Secondly, the test used a simple sleep to wait for the temporary file to
be created.  This is obviously problematic, because on slow machines (or
with valgrind, CLOBBER_CACHE_ALWAYS etc.) it may take a while to create
the temporary file.  But we also want the tests run reasonably fast.
Fixed by instead relying on a UNIQUE constraint, blocking the query that
created the temporary file.

Author: Euler Taveira
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

Only "IMPORT" was showing as result of the completion, while IMPORT
FOREIGN SCHEMA is the only command using this keyword in first
position.  This changes the completion to show the full command name
instead of just "IMPORT".

Reviewed-by: Georgios Kokolatos, Julien Rouhaud
Discussion: https://postgr.es/m/YFL6JneBiuMWYyoh@paquier.xyz

Our coding convention requires this macro's result to be assigned
back to the original List variable.  In this usage, since the
List could not become empty, there was no actual bug --- but
some compilers warned about it.  Oversight in be45be9c.

Discussion: https://postgr.es/m/35077b31-2d62-1e31-0e2e-ddb52d590b73@enterprisedb.com

With grouping sets, it's possible that some of the grouping sets are
duplicate.  This is especially common with CUBE and ROLLUP clauses. For
example GROUP BY CUBE (a,b), CUBE (b,c) is equivalent to

  GROUP BY GROUPING SETS (
    (a, b, c),
    (a, b, c),
    (a, b, c),
    (a, b),
    (a, b),
    (a, b),
    (a),
    (a),
    (a),
    (c, a),
    (c, a),
    (c, a),
    (c),
    (b, c),
    (b),
    ()
  )

Some of the grouping sets are calculated multiple times, which is mostly
unnecessary.  This commit implements a new GROUP BY DISTINCT feature, as
defined in the SQL standard, which eliminates the duplicate sets.

Author: Vik Fearing
Reviewed-by: Erik Rijkers, Georgios Kokolatos, Tomas Vondra
Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org