It was previously possible to have the launcher re-execute its main loop
before shutting down if some other signal was received or an error
occurred after getting SIGTERM, as reported by Qingqing Zhou.
While investigating, Tom Lane further noticed that if autovacuum had
been disabled in the config file, it would misbehave by trying to start
a new worker instead of bailing out immediately -- it would consider
itself as invoked in emergency mode.
Fix both problems by checking the shutdown flag in a few more places.
These problems have existed since autovacuum was introduced, so
backpatch all the way back.
While gcc doesn't complain if you declare a function "static" and then
define it not-static, other compilers do; and in any case the code is
highly misleading this way. Add the missing "static" keywords to a
couple of recent patches. Per buildfarm member pademelon.
Considering the number of cases in which "unused" command line arguments
are silently ignored by compilers, it's fairly astonishing that anybody
thought this warning was useful; it's certainly nothing but an annoyance
when building Postgres. One such case is that neither gcc nor clang
complain about unrecognized -Wno-foo switches, making it more difficult
to figure out whether the switch does anything than one could wish.
Back-patch to 9.3, which is as far back as the patch applies conveniently
(we'd have to back-patch PGAC_PROG_CC_VAR_OPT to go further, and it doesn't
seem worth that).
Previously we would re-use input subexpressions in all expression trees
attached to a Join plan node. However, if it's an outer join and the
subexpression appears in the nullable-side input, this is potentially
incorrect for apparently-matching subexpressions that came from above
the outer join (ie, targetlist and qpqual expressions), because the
executor will treat the subexpression value as NULL when maybe it should
not be.
The case is fairly hard to hit because (a) you need a non-strict
subexpression (else NULL is correct), and (b) we don't usually compute
expressions in the outputs of non-toplevel plan nodes. But we might do
so if the expressions are sort keys for a mergejoin, for example.
Probably in the long run we should make a more explicit distinction between
Vars appearing above and below an outer join, but that will be a major
planner redesign and not at all back-patchable. For the moment, just hack
set_join_references so that it will not match any non-Var expressions
coming from nullable inputs to expressions that came from above the join.
(This is somewhat overkill, in that a strict expression could still be
matched, but it doesn't seem worth the effort to check that.)
Per report from Qingqing Zhou. The added regression test case is based
on his example.
This has been broken for a very long time, so back-patch to all active
branches.
Commit ed9cc2b5df59fdbc50cce37399e26b03ab2c1686 made it unnecessary to pass
start_nblkno to _hash_splitbucket(), and for that matter unnecessary to
have the internal nblkno variable either. My compiler didn't complain
about that, but some did. I also rearranged the use of oblkno a bit to
make that case more parallel.
Report and initial patch by Petr Jelinek, rearranged a bit by me.
Back-patch to all branches, like the previous patch.
psql was already accepting conninfo strings as the first parameter in
\connect, but the way it worked wasn't sane; some of the other
parameters would get the previous connection's values, causing it to
connect to a completely unexpected server or, more likely, not finding
any server at all because of completely wrong combinations of
parameters.
Fix by explicitely checking for a conninfo-looking parameter in the
dbname position; if one is found, use its complete specification rather
than mix with the other arguments. Also, change tab-completion to not
try to complete conninfo/URI-looking "dbnames" and document that
conninfos are accepted as first argument.
There was a weak consensus to backpatch this, because while the behavior
of using the dbname as a conninfo is nowhere documented for \connect, it
is reasonable to expect that it works because it does work in many other
contexts. Therefore this is backpatched all the way back to 9.0.
To implement this, routines previously private to libpq have been
duplicated so that psql can decide what looks like a conninfo/URI
string. In back branches, just duplicate the same code all the way back
to 9.2, where URIs where introduced; 9.0 and 9.1 have a simpler version.
In master, the routines are moved to src/common and renamed.
Author: David Fetter, Andrew Dunstan. Some editorialization by me
(probably earning a Gierth's "Sloppy" badge in the process.)
Reviewers: Andrew Gierth, Erik Rijkers, Pavel Stěhule, Stephen Frost,
Robert Haas, Andrew Dunstan.
You're required to write either RANGE or ROWS to start a frame clause,
but the documentation incorrectly implied this is optional. Noted by
David Johnston.
As with initdb these programs need to run with a restricted token, and
if they don't pg_upgrade will fail when run as a user with Adminstrator
privileges.
Backpatch to all live branches. On the development branch the code is
reorganized so that the restricted token code is now in a single
location. On the stable bramches a less invasive change is made by
simply copying the relevant code to pg_upgrade.c and pg_resetxlog.c.
Patches and bug report from Muhammad Asif Naeem, reviewed by Michael
Paquier, slightly edited by me.
_hash_splitbucket() obtained the base page of the new bucket by calling
_hash_getnewbuf(), but it held no exclusive lock that would prevent some
other process from calling _hash_getnewbuf() at the same time. This is
contrary to _hash_getnewbuf()'s API spec and could in fact cause failures.
In practice, we must only call that function while holding write lock on
the hash index's metapage.
An additional problem was that we'd already modified the metapage's bucket
mapping data, meaning that failure to extend the index would leave us with
a corrupt index.
Fix both issues by moving the _hash_getnewbuf() call to just before we
modify the metapage in _hash_expandtable().
Unfortunately there's still a large problem here, which is that we could
also incur ENOSPC while trying to get an overflow page for the new bucket.
That would leave the index corrupt in a more subtle way, namely that some
index tuples that should be in the new bucket might still be in the old
one. Fixing that seems substantially more difficult; even preallocating as
many pages as we could possibly need wouldn't entirely guarantee that the
bucket split would complete successfully. So for today let's just deal
with the base case.
Per report from Antonin Houska. Back-patch to all active branches.
Slow functions in index expressions might cause this loop to take long
enough to make it worth being cancellable. Probably it would be enough
to call CHECK_FOR_INTERRUPTS here, but for consistency with other
per-sample-row loops in this file, let's use vacuum_delay_point.
Report and patch by Jeff Janes. Back-patch to all supported branches.
ExecOpenScanRelation assumed that any relation listed in the ExecRowMark
list has been locked by InitPlan; but this is not true if the rel's
markType is ROW_MARK_COPY, which is possible if it's a foreign table.
In most (possibly all) cases, failure to acquire a lock here isn't really
problematic because the parser, planner, or plancache would have taken the
appropriate lock already. In principle though it might leave us vulnerable
to working with a relation that we hold no lock on, and in any case if the
executor isn't depending on previously-taken locks otherwise then it should
not do so for ROW_MARK_COPY relations.
Noted by Etsuro Fujita. Back-patch to all active versions, since the
inconsistency has been there a long time. (It's almost certainly
irrelevant in 9.0, since that predates foreign tables, but the code's
still wrong on its own terms.)
It's all very well to claim that a simplistic sort is fast in easy
cases, but O(N^2) in the worst case is not good ... especially if the
worst case is as easy to hit as "descending order input". Replace that
bit with our standard qsort.
Per bug #12866 from Maksym Boguk. Back-patch to all active branches.
GNU readline defines the return value of write_history() as "zero if OK,
else an errno code". libedit's version of that function used to have a
different definition (to wit, "-1 if error, else the number of lines
written to the file"). We tried to work around that by checking whether
errno had become nonzero, but this method has never been kosher according
to the published API of either library. It's reportedly completely broken
in recent Ubuntu releases: psql bleats about "No such file or directory"
when saving ~/.psql_history, even though the write worked fine.
However, libedit has been following the readline definition since somewhere
around 2006, so it seems all right to finally break compatibility with
ancient libedit releases and trust that the return value is what readline
specifies. (I'm not sure when the various Linux distributions incorporated
this fix, but I did find that OS X has been shipping fixed versions since
10.5/Leopard.)
If anyone is still using such an ancient libedit, they will find that psql
complains it can't write ~/.psql_history at exit, even when the file was
written correctly. This is no worse than the behavior we're fixing for
current releases.
Back-patch to all supported branches.
The message tries to tell the replication apply delay which fails if
the first WAL record is not applied yet. Fix is, instead of telling
overflowed minus numeric, showing "N/A" which indicates that the delay
data is not yet available. Problem reported by me and patch by
Fabrízio de Royes Mello.
Back patched to 9.4, 9.3 and 9.2 stable branches (9.1 and 9.0 do not
have the debug message).
The ROW_MARK_COPY path in EvalPlanQualFetchRowMarks() was just setting
tableoid to InvalidOid, I think on the assumption that the referenced
RTE must be a subquery or other case without a meaningful OID. However,
foreign tables also use this code path, and they do have meaningful
table OIDs; so failure to set the tuple field can lead to user-visible
misbehavior. Fix that by fetching the appropriate OID from the range
table.
There's still an issue about whether CTID can ever have a meaningful
value in this case; at least with postgres_fdw foreign tables, it does.
But that is a different problem that seems to require a significantly
different patch --- it's debatable whether postgres_fdw really wants to
use this code path at all.
Simplified version of a patch by Etsuro Fujita, who also noted the
problem to begin with. The issue can be demonstrated in all versions
having FDWs, so back-patch to 9.1.
This is a possibly-vain effort to silence a Coverity warning about
bogus endianness dependency. The code's fine, because it takes care
of endianness issues for itself, but Coverity sees an int64 being
passed to an int* argument and not unreasonably suspects something's
wrong. I'm not sure if putting the void* cast in the way will shut it
up; but it can't hurt and seems better from a documentation standpoint
anyway, since the pointer is not used as an int* in this code path.
Just for a bit of additional safety, verify that the result length
is 8 bytes as expected.
Back-patch to 9.3 where the code in question was added.
The SGML docs claimed that 1-byte integers could be sent or received with
the "isint" options, but no such behavior has ever been implemented in
pqGetInt() or pqPutInt(). The in-code documentation header for PQfn() was
even less in tune with reality, and the code itself used parameter names
matching neither the SGML docs nor its libpq-fe.h declaration. Do a bit
of additional wordsmithing on the SGML docs while at it.
Since the business about 1-byte integers is a clear documentation bug,
back-patch to all supported branches.
Commit 7b583b20b1c95acb621c71251150beef958bb603 created an unnecessary
dump failure hazard by applying pg_get_function_identity_arguments()
to every function in the database, even those that won't get dumped.
This could result in snapshot-related problems if concurrent sessions are,
for example, creating and dropping temporary functions, as noted by Marko
Tiikkaja in bug #12832. While this is by no means pg_dump's only such
issue with concurrent DDL, it's unfortunate that we added a new failure
mode for cases that used to work, and even more so that the failure was
created for basically cosmetic reasons (ie, to sort overloaded functions
more deterministically).
To fix, revert that patch and instead sort function arguments using
information that pg_dump has available anyway, namely the names of the
argument types. This will produce a slightly different sort ordering for
overloaded functions than the previous coding; but applying strcmp
directly to the output of pg_get_function_identity_arguments really was
a bit odd anyway. The sorting will still be name-based and hence
independent of possibly-installation-specific OID assignments. A small
additional benefit is that sorting now works regardless of server version.
Back-patch to 9.3, where the previous commit appeared.
We were using "user mapping for user XYZ" as description for user mappings, but
that's ambiguous because users can have mappings on multiple foreign
servers; therefore change it to "for user XYZ on server UVW" instead.
Object identities for user mappings are also updated in the same way, in
branches 9.3 and above.
The incomplete description string was introduced together with the whole
SQL/MED infrastructure by commit cae565e503 of 8.4 era, so backpatch all
the way back.
Since 9.1, we've provided extensions with a way to denote
"configuration" tables- tables created by an extension which the user
may modify. By marking these as "configuration" tables, the extension
is asking for the data in these tables to be pg_dump'd (tables which
are not marked in this way are assumed to be entirely handled during
CREATE EXTENSION and are not included at all in a pg_dump).
Unfortunately, pg_dump neglected to consider foreign key relationships
between extension configuration tables and therefore could end up
trying to reload the data in an order which would cause FK violations.
This patch teaches pg_dump about these dependencies, so that the data
dumped out is done so in the best order possible. Note that there's no
way to handle circular dependencies, but those have yet to be seen in
the wild.
The release notes for this should include a caution to users that
existing pg_dump-based backups may be invalid due to this issue. The
data is all there, but restoring from it will require extracting the
data for the configuration tables and then loading them in the correct
order by hand.
Discussed initially back in bug #6738, more recently brought up by
Gilles Darold, who provided an initial patch which was further reworked
by Michael Paquier. Further modifications and documentation updates
by me.
Back-patch to 9.1 where we added the concept of extension configuration
tables.
When the library already exists in the build directory, "ar" preserves
members not named on its command line. This mattered when, for example,
a "configure" rerun dropped a file from $(LIBOBJS). libpgport carried
the obsolete member until "make clean". Back-patch to 9.0 (all
supported versions).
Part of the intent of the parameterized-path mechanism was to handle
star-schema queries efficiently, but some overly-restrictive search
limiting logic added in commit e2fa76d80ba571d4de8992de6386536867250474
prevented such cases from working as desired. Fix that and add a
regression test about it. Per gripe from Marc Cousin.
This is arguably a bug rather than a new feature, so back-patch to 9.2
where parameterized paths were introduced.
Up to now RecordTransactionCommit() waited for WAL to be flushed (if
synchronous_commit != off) and to be synchronously replicated (if
enabled), even if a transaction did not have a xid assigned. The primary
reason for that is that sequence's nextval() did not assign a xid, but
are worthwhile to wait for on commit.
This can be problematic because sometimes read only transactions do
write WAL, e.g. HOT page prune records. That then could lead to read only
transactions having to wait during commit. Not something people expect
in a read only transaction.
This lead to such strange symptoms as backends being seemingly stuck
during connection establishment when all synchronous replicas are
down. Especially annoying when said stuck connection is the standby
trying to reconnect to allow syncrep again...
This behavior also is involved in a rather complicated <= 9.4 bug where
the transaction started by catchup interrupt processing waited for
syncrep using latches, but didn't get the wakeup because it was already
running inside the same overloaded signal handler. Fix the issue here
doesn't properly solve that issue, merely papers over the problems. In
9.5 catchup interrupts aren't processed out of signal handlers anymore.
To fix all this, make nextval() acquire a top level xid, and only wait for
transaction commit if a transaction both acquired a xid and emitted WAL
records. If only a xid has been assigned we don't uselessly want to
wait just because of writes to temporary/unlogged tables; if only WAL
has been written we don't want to wait just because of HOT prunes.
The xid assignment in nextval() is unlikely to cause overhead in
real-world workloads. For one it only happens SEQ_LOG_VALS/32 values
anyway, for another only usage of nextval() without using the result in
an insert or similar is affected.
Discussion: 20150223165359.GF30784@awork2.anarazel.de,
369698E947874884A77849D8FE3680C2@maumau,
5CF4ABBA67674088B3941894E22A0D25@maumau
Per complaint from maumau and Thom Brown
Backpatch all the way back; 9.0 doesn't have syncrep, but it seems
better to be consistent behavior across all maintained branches.
"RETURN SQLERRM" prompted plpgsql_exec_function() to read from freed
memory. Back-patch to 9.0 (all supported versions). Little code ran
between the premature free and the read, so non-assert builds are
unlikely to witness user-visible consequences.
The "simple" path for printing VALUES clauses doesn't work if we need
to attach nondefault column aliases, because there's noplace to do that
in the minimal VALUES() syntax. So modify get_simple_values_rte() to
detect nondefault aliases and treat that as a non-simple case. This
further exposes that the "non-simple" path never actually worked;
it didn't produce valid syntax. Fix that too. Per bug #12789 from
Curtis McEnroe, and analysis by Andrew Gierth.
Back-patch to all supported branches. Before 9.3, this also requires
back-patching the part of commit 092d7ded29f36b0539046b23b81b9f0bf2d637f1
that created get_simple_values_rte() to begin with; inserting the extra
test into the old factorization of that logic would've been too messy.
When LockBufferForCleanup() has to wait for getting a cleanup lock on a
buffer it does so by setting a flag in the buffer header and then wait
for other backends to signal it using ProcWaitForSignal().
Unfortunately LockBufferForCleanup() missed that ProcWaitForSignal() can
return for other reasons than the signal it is hoping for. If such a
spurious signal arrives the wait flags on the buffer header will still
be set. That then triggers "ERROR: multiple backends attempting to wait
for pincount 1".
The fix is simple, unset the flag if still set when retrying. That
implies an additional spinlock acquisition/release, but that's unlikely
to matter given the cost of waiting for a cleanup lock. Alternatively
it'd have been possible to move responsibility for maintaining the
relevant flag to the waiter all together, but that might have had
negative consequences due to possible floods of signals. Besides being
more invasive.
This looks to be a very longstanding bug. The relevant code in
LockBufferForCleanup() hasn't changed materially since its introduction
and ProcWaitForSignal() was documented to return for unrelated reasons
since 8.2. The master only patch series removing ImmediateInterruptOK
made it much easier to hit though, as ProcSendSignal/ProcWaitForSignal
now uses a latch shared with other tasks.
Per discussion with Kevin Grittner, Tom Lane and me.
Backpatch to all supported branches.
Discussion: 11553.1423805224@sss.pgh.pa.us
If libpq output buffer is full, pqSendSome() function tries to drain any
incoming data. This avoids deadlock, if the server e.g. sends a lot of
NOTICE messages, and blocks until we read them. However, pqSendSome() only
did that in blocking mode. In non-blocking mode, the deadlock could still
happen.
To fix, take a two-pronged approach:
1. Change the documentation to instruct that when PQflush() returns 1, you
should wait for both read- and write-ready, and call PQconsumeInput() if it
becomes read-ready. That fixes the deadlock, but applications are not going
to change overnight.
2. In pqSendSome(), drain the input buffer before returning 1. This
alleviates the problem for applications that only wait for write-ready. In
particular, a slow but steady stream of NOTICE messages during COPY FROM
STDIN will no longer cause a deadlock. The risk remains that the server
attempts to send a large burst of data and fills its output buffer, and at
the same time the client also sends enough data to fill its output buffer.
The application will deadlock if it goes to sleep, waiting for the socket
to become write-ready, before the server's data arrives. In practice,
NOTICE messages and such that the server might be sending are usually
short, so it's highly unlikely that the server would fill its output buffer
so quickly.
Backpatch to all supported versions.
After finding an "=" character, the pointer was advanced twice when it
should only advance once. This is harmless as long as the value after "="
has at least one character; but if it doesn't, we'd miss the terminator
character and include too much in the value.
In principle this could lead to reading off the end of memory. It does not
seem worth treating as a security issue though, because it would happen on
client side, and besides client logic that's taking conninfo strings from
untrusted sources has much worse security problems than this.
Report and patch received off-list from Thomas Fanghaenel.
Back-patch to 9.2 where the faulty code was introduced.
cfopen() and cfopen_write() failed to pass the compression level through
to zlib, so that you always got the default compression level if you got
any at all.
In passing, also fix these and related functions so that the correct errno
is reliably returned on failure; the original coding supposes that free()
cannot change errno, which is untrue on at least some platforms.
Per bug #12779 from Christoph Berg. Back-patch to 9.1 where the faulty
code was introduced.
Michael Paquier
In investigating yesterday's crash report from Hugo Osvaldo Barrera, I only
looked back as far as commit f3aec2c7f51904e7 where the breakage occurred
(which is why I thought the IPv4-in-IPv6 business was undocumented). But
actually the logic dates back to commit 3c9bb8886df7d56a and was simply
broken by erroneous refactoring in the later commit. A bit of archives
excavation shows that we added the whole business in response to a report
that some 2003-era Linux kernels would report IPv4 connections as having
IPv4-in-IPv6 addresses. The fact that we've had no complaints since 9.0
seems to be sufficient confirmation that no modern kernels do that, so
let's just rip it all out rather than trying to fix it.
Do this in the back branches too, thus essentially deciding that our
effective behavior since 9.0 is correct. If there are any platforms on
which the kernel reports IPv4-in-IPv6 addresses as such, yesterday's fix
would have made for a subtle and potentially security-sensitive change in
the effective meaning of IPv4 pg_hba.conf entries, which does not seem like
a good thing to do in minor releases. So let's let the post-9.0 behavior
stand, and change the documentation to match it.
In passing, I failed to resist the temptation to wordsmith the description
of pg_hba.conf IPv4 and IPv6 address entries a bit. A lot of this text
hasn't been touched since we were IPv4-only.
Avoid losing errno if readdir() fails and closedir() works. Consistently
return 4 rather than 3 if both a lost+found directory and other files are
found, rather than returning one value or the other depending on the
order of the directory listing. Update comments to match the actual
behavior.
These oversights date to commits 6f03927fce038096f53ca67eeab9adb24938f8a6
and 17f15239325a88581bb4f9cf91d38005f1f52d69.
Marco Nenciarini
The previous coding copied garbage into a local variable, pretty much
ensuring that the intended test of an IPv6 connection address against a
promoted IPv4 address from pg_hba.conf would never match. The lack of
field complaints likely indicates that nobody realized this was supposed
to work, which is unsurprising considering that no user-facing docs suggest
it should work.
In principle this could have led to a SIGSEGV due to reading off the end of
memory, but since the source address would have pointed to somewhere in the
function's stack frame, that's quite unlikely. What led to discovery of
the bug is Hugo Osvaldo Barrera's report of a crash after an OS upgrade,
which is probably because he is now running a system in which memcpy raises
abort() upon detecting overlapping source and destination areas. (You'd
have to additionally suppose some things about the stack frame layout to
arrive at this conclusion, but it seems plausible.)
This has been broken since the code was added, in commit f3aec2c7f51904e7,
so back-patch to all supported branches.
In commit bf7ca15875988a88e97302e012d7c4808bef3ea9 I introduced an
assumption that an RTE referenced by a whole-row Var must have a valid eref
field. This is false for RTEs constructed by DoCopy, and there are other
places taking similar shortcuts. Perhaps we should make all those places
go through addRangeTableEntryForRelation or its siblings instead of having
ad-hoc logic, but the most reliable fix seems to be to make the new code in
ExecEvalWholeRowVar cope if there's no eref. We can reasonably assume that
there's no need to insert column aliases if no aliases were provided.
Add a regression test case covering this, and also verifying that a sane
column name is in fact available in this situation.
Although the known case only crashes in 9.4 and HEAD, it seems prudent to
back-patch the code change to 9.2, since all the ingredients for a similar
failure exist in the variant patch applied to 9.3 and 9.2.
Per report from Jean-Pierre Pelletier.
pg_database.datfrozenxid and pg_database.datminmxid were not preserved
for the 'postgres' and 'template1' databases. This could cause missing
clog file errors on access to user tables and indexes after upgrades in
these databases.
Backpatch through 9.0
This omission leaked one PGresult per WAL streaming cycle, which possibly
would never be enough to notice in the real world, but it's still a leak.
Per Coverity. Back-patch to 9.3 where the error was introduced.
We'd leak the ident_serv data structure if the second pg_getaddrinfo_all
(the one for the local address) failed. This is not of great consequence
because a failure return here just leads directly to backend exit(), but
if this function is going to try to clean up after itself at all, it should
not have such holes in the logic. Try to fix it in a future-proof way by
having all the failure exits go through the same cleanup path, rather than
"optimizing" some of them.
Per Coverity. Back-patch to 9.2, which is as far back as this patch
applies cleanly.
We already had one go at this issue in commit d73b7f973db5ec7e, but we
failed to notice that buildACLCommands also leaked several PQExpBuffers
along with a simply malloc'd string. This time let's try to make the
fix a bit more future-proof by eliminating the separate exit path.
It's still not exactly critical because pg_dump will curl up and die on
failure; but since the amount of the potential leak is now several KB,
it seems worth back-patching as far as 9.2 where the previous fix landed.
Per Coverity, which evidently is smarter than clang's static analyzer.
When ecpg was rewritten to the new protocol version not all variable types
were corrected. This patch rewrites the code for these types to fix that. It
also fixes the documentation to correctly tell the status of array handling.
Back in 2003 we had a discussion about how to decide which casts to dump.
At the time pg_dump really only considered an object's containing schema
to decide what to dump (ie, dump whatever's not in pg_catalog), and so
we chose a complicated idea involving whether the underlying types were to
be dumped (cf commit a6790ce85752b67ad994f55fdf1a450262ccc32e). But users
are allowed to create casts between built-in types, and we failed to dump
such casts. Let's get rid of that heuristic, which has accreted even more
ugliness since then, in favor of just looking at the cast's OID to decide
if it's a built-in cast or not.
In passing, also fix some really ancient code that supposed that it had to
manufacture a dependency for the cast on its cast function; that's only
true when dumping from a pre-7.3 server. This just resulted in some wasted
cycles and duplicate dependency-list entries with newer servers, but we
might as well improve it.
Per gripes from a number of people, most recently Greg Sabino Mullane.
Back-patch to all supported branches.
Back in commit 400e2c934457bef4bc3cc9a3e49b6289bd761bc0 I rewrote GEQO's
gimme_tree function to improve its heuristic for modifying the given tour
into a legal join order. In what can only be called a fit of hubris,
I supposed that this new heuristic would *always* find a legal join order,
and ripped out the old logic that allowed gimme_tree to sometimes fail.
The folly of this is exposed by bug #12760, in which the "greedy" clumping
behavior of merge_clump() can lead it into a dead end which could only be
recovered from by un-clumping. We have no code for that and wouldn't know
exactly what to do with it if we did. Rather than try to improve the
heuristic rules still further, let's just recognize that it *is* a
heuristic and probably must always have failure cases. So, put back the
code removed in the previous commit to allow for failure (but comment it
a bit better this time).
It's possible that this code was actually fully correct at the time and
has only been broken by the introduction of LATERAL. But having seen this
example I no longer have much faith in that proposition, so back-patch to
all supported branches.
