PostgreSQL Weekly News - November 28, 2021

Posted on 2021-11-29 by PWN

PostgreSQL Weekly News - November 28, 2021

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Applied Patches

Peter Geoghegan pushed:

  • Remove lazy_scan_heap parallel VACUUM comment block. This doesn't belong next to very high level discussion of the tasks that lazy_scan_heap performs. There is already a similar, longer comment block at the top of vacuumlazy.c that mentions lazy_scan_heap directly.

  • Go back to considering HOT on pages marked full. Commit 2fd8685e7f simplified the checking of modified attributes that takes place within heap_update(). This included a micro-optimization affecting pages marked PD_PAGE_FULL: don't even try to use HOT to save a few cycles on determining HOT safety. The assumption was that it won't work out this time around, since it can't have worked out last time around. Remove the micro-optimization. It could only ever save cycles that are consumed by the vast majority of heap_update() calls, which hardly seems worth the added complexity. It also seems quite possible that there are workloads that will do worse over time by repeated application of the micro-optimization, despite saving some cycles on average, in the short term. Author: Peter Geoghegan Reviewed-By: Álvaro Herrera Discussion:

  • Update high level vacuumlazy.c comments. Update vacuumlazy.c file header comments (as well as comments above the lazy_scan_heap function) that were largely written before the introduction of the HOT optimization, when lazy_scan_heap did far less, and didn't actually prune during its initial heap pass. Since lazy_scan_heap now outsources far more work to lower level functions, it makes sense to introduce the function by talking about the high level invariant that dictates the order in which each phase takes place. Also deemphasize the case where we run out of memory for TIDs, since delaying that discussion makes it easier to talk about issues of central importance. Finally, remove discussion of parallel VACUUM from header comments. These don't add much, and are in the wrong place.

  • vacuumlazy.c: prefer the term "cleanup lock". The term "super-exclusive lock" is an acceptable synonym of "cleanup lock". Even still, switching from one term to the other in the same file is confusing. Standardize on "cleanup lock" within vacuumlazy.c. Per a complaint from Andres Freund.

Fujii Masao pushed:

Peter Eisentraut pushed:

Álvaro Herrera pushed:

Tom Lane pushed:

  • Probe $PROVE not $PERL while checking for modules needed by TAP tests. Normally "prove" and "perl" come from the same Perl installation, but we support the case where they don't (mainly because the MSys buildfarm animals need this). In that case, AX_PROG_PERL_MODULES is completely the wrong thing to use, because it's checking what "perl" has. Instead, make a little TAP test script including the required modules, and run that under "prove". We don't need ax_prog_perl_modules.m4 at all after this change, so remove it. Back-patch to all supported branches, for the buildfarm's benefit. (In v10, this also back-patches the effects of commit 264eb03aa.) Andrew Dunstan and Tom Lane, per an observation by Noah Misch Discussion:

  • Fix pg_dump --inserts mode for generated columns with dropped columns. If a table contains a generated column that's preceded by a dropped column, dumpTableData_insert failed to account for the dropped column, and would emit DEFAULT placeholder(s) in the wrong column(s). This resulted in failures at restore time. The default COPY code path did not have this bug, likely explaining why it wasn't noticed sooner. While we're fixing this, we can be a little smarter about the situation: (1) avoid unnecessarily fetching the values of generated columns, (2) omit generated columns from the output, too, if we're using --column-inserts. While these modes aren't expected to be as high-performance as the COPY path, we might as well be as efficient as we can; it doesn't add much complexity. Per report from Дмитрий Иванов. Back-patch to v12 where generated columns came in. Discussion:

  • Pacify perlcritic. Per buildfarm.

  • Adjust pg_dump's priority ordering for casts. When a stored expression depends on a user-defined cast, the backend records the dependency as being on the cast's implementation function --- or indeed, if there's no cast function involved but just RelabelType or CoerceViaIO, no dependency is recorded at all. This is problematic for pg_dump, which is at risk of dumping things in the wrong order leading to restore failures. Given the lack of previous reports, the risk isn't that high, but it can be demonstrated if the cast is used in some view whose rowtype is then used as an input or result type for some other function. (That results in the view getting hoisted into the functions portion of the dump, ahead of the cast.) A logically bulletproof fix for this would require including the cast's OID in the parsed form of the expression, whence it could be extracted by dependency.c, and then the stored dependency would force pg_dump to do the right thing. Such a change would be fairly invasive, and certainly not back-patchable. Moreover, since we'd prefer that an expression using cast syntax be equal() to one doing the same thing by explicit function call, the cast OID field would have to have special ignored-by-comparisons semantics, making things messy. So, let's instead fix this by a very simple hack in pg_dump: change the object-type priority order so that casts are initially sorted before functions, immediately after types. This fixes the problem in a fairly direct way for casts that have no implementation function. For those that do, the implementation function will be hoisted to just before the cast by the dependency sorting step, so that we still have a valid dump order. (I'm not sure that this provides a full guarantee of no problems; but since it's been like this for many years without any previous reports, this is probably enough to fix it in practice.) Per report from Дмитрий Иванов. Back-patch to all supported branches. Discussion:

  • Doc: improve documentation about nextval()/setval(). Clarify that the results of nextval and setval are not guaranteed persistent until the calling transaction commits. Some people seem to have drawn the opposite conclusion from the statement that these functions are never rolled back, so re-word to avoid saying it quite that way. Discussion:

Michaël Paquier pushed:

  • Add SQL functions to monitor the directory contents of replication slots. This commit adds a set of functions able to look at the contents of various paths related to replication slots: - pg_ls_logicalsnapdir, for pg_logical/snapshots/ - pg_ls_logicalmapdir, for pg_logical/mappings/ - pg_ls_replslotdir, for pg_replslot/<slot_name>/ These are intended to be used by monitoring tools. Unlike pg_ls_dir(), execution permission can be granted to non-superusers. Roles members of pg_monitor gain have access to those functions. Bump catalog version. Author: Bharath Rupireddy Reviewed-by: Nathan Bossart, Justin Pryzby Discussion:

  • Add support for Visual Studio 2022 in build scripts. Documentation and any code paths related to VS are updated to keep the whole consistent. Similarly to 2017 and 2019, the version of VS and the version of nmake that we use to determine which code paths to use for the build are still inconsistent in their own way. Backpatch down to 10, so as buildfarm members are able to use this new version of Visual Studio on all the stable branches supported. Author: Hans Buschmann Discussion: Backpatch-through: 10

  • Remove useless LZ4 system call on failure when writing file header. If an error occurs when writing the LZ4 file header, LZ4F_compressEnd() was called in the error code path of write(), followed by LZ4F_freeCompressionContext() to finish the cleanup. The code as-is was not broken, but the LZ4F_compressEnd() proves to not be necessary as there are no contents to flush at this stage, so remove it. Per gripe from Jeevan Ladhe and Robert Haas. Discussion:

  • Fix fstat() emulation on Windows with standard streams. The emulation of fstat() in win32stat.c caused two issues with the existing in-core callers, failing on EINVAL when using a stream as argument: - psql's \copy would crash when using a stream. - pg_recvlogical would fail with -f -. The tests in copyselect.sql from the main test suite covers the first case, and there is a TAP test for the second case. However, in both cases, as the standard streams are always redirected, automated tests did not notice those issues, requiring a terminal on Windows to be reproducible. This issue has been introduced in bed9075, and the origin of the problem is that GetFileInformationByHandle() does not work directly on streams, so this commit adds an extra code path to emulate and return a set of stats that match best with the reality. Note that redirected streams rely on handles that can be queried with GetFileInformationByHandle(), but we can rely on GetFinalPathNameByHandleA() to detect this case. Author: Dmitry Koval, Juan José Santamaría Flecha Discussion: Backpatch-through: 14

  • Block ALTER TABLE .. DROP NOT NULL on columns in replica identity index. Replica identities that depend directly on an index rely on a set of properties, one of them being that all the columns defined in this index have to be marked as NOT NULL. There was a hole in the logic with ALTER TABLE DROP NOT NULL, where it was possible to remove the NOT NULL property of a column part of an index used as replica identity, so block it to avoid problems with logical decoding down the road. The same check was already done columns part of a primary key, so the fix is straight-forward. Author: Haiying Tang, Hou Zhijie Reviewed-by: Dilip Kumar, Michael Paquier Discussion: Backpatch-through: 10

David Rowley pushed:

  • Allow Memoize to operate in binary comparison mode. Memoize would always use the hash equality operator for the cache key types to determine if the current set of parameters were the same as some previously cached set. Certain types such as floating points where -0.0 and +0.0 differ in their binary representation but are classed as equal by the hash equality operator may cause problems as unless the join uses the same operator it's possible that whichever join operator is being used would be able to distinguish the two values. In which case we may accidentally return in the incorrect rows out of the cache. To fix this here we add a binary mode to Memoize to allow it to the current set of parameters to previously cached values by comparing bit-by-bit rather than logically using the hash equality operator. This binary mode is always used for LATERAL joins and it's used for normal joins when any of the join operators are not hashable. Reported-by: Tom Lane Author: David Rowley Discussion: Backpatch-through: 14, where Memoize was added

  • Flush Memoize cache when non-key parameters change. It's possible that a subplan below a Memoize node contains a parameter from above the Memoize node. If this parameter changes then cache entries may become out-dated due to the new parameter value. Previously Memoize was mistakenly not aware of this. We fix this here by flushing the cache whenever a parameter that's not part of the cache key changes. Bug: #17213 Reported by: Elvis Pranskevichus Author: David Rowley Discussion: Backpatch-through: 14, where Memoize was added

  • Revert "Flush Memoize cache when non-key parameters change". This reverts commit 1050048a315790a505465bfcceb26eaf8dbc7e2e.

  • Flush Memoize cache when non-key parameters change, take 2. It's possible that a subplan below a Memoize node contains a parameter from above the Memoize node. If this parameter changes then cache entries may become out-dated due to the new parameter value. Previously Memoize was mistakenly not aware of this. We fix this here by flushing the cache whenever a parameter that's not part of the cache key changes. Bug: #17213 Reported by: Elvis Pranskevichus Author: David Rowley Discussion: Backpatch-through: 14, where Memoize was added

Amit Kapila pushed:

Robert Haas pushed:

  • Fix corner-case failure to detect improper timeline switch. rescanLatestTimeLine() contains a guard against switching to a timeline that forked off from the current one prior to the current recovery point, but that guard does not work if the timeline switch occurs before the first WAL recod (which must be the checkpoint record) is read. Without this patch, an improper timeline switch is therefore possible in such cases. This happens because rescanLatestTimeLine() relies on the global variable EndRecPtr to understand the current position of WAL replay. However, EndRecPtr at this point in the code contains the endpoint of the last-replayed record, not the startpoint or endpoint of the record being replayed now. Thus, before any records have been replayed, it's zero, which causes the sanity check to always pass. To fix, pass down the correct timeline explicitly. The EndRecPtr value we want is the one from the xlogreader, which will be the starting position of the record we're about to try to read, rather than the global variable, which is the ending position of the last record we successfully read. They're usually the same, but not in the corner case described here. No back-patch, because in v14 and earlier branhes, we were using the wrong TLI here as well as the wrong LSN. In master, that was fixed by commit 4a92a1c3d1c361ffb031ed05bf65b801241d7cdd, but that and it's prerequisite patches are too invasive to back-patch for such a minor issue. Patch by me, reviewed by Amul Sul. Discussion:

  • xlog.c: Remove global variables ReadRecPtr and EndRecPtr. In most places, the variables necessarily store the same value as the eponymous members of the XLogReaderState that we use during WAL replay, because ReadRecord() assigns the values from the structure members to the global variables just after XLogReadRecord() returns. However, XLogBeginRead() adjusts the structure members but not the global variables, so after XLogBeginRead() and before the completion of XLogReadRecord() the values can differ. Otherwise, they must be identical. According to my analysis, the only place where either variable is referenced at a point where it might not have the same value as the structure member is the refrence to EndRecPtr within XLogPageRead. Therefore, at every other place where we are using the global variable, we can just switch to using the structure member instead, and remove the global variable. However, we can, and in fact should, do this in XLogPageRead() as well, because at that point in the code, the global variable will actually store the start of the record we want to read - either because it's where the last WAL record ended, or because the read position has been changed using XLogBeginRead since the last record was read. The structure member, on the other hand, will already have been updated to point to the end of the record we just read. Elsewhere, the latter is what we use as an argument to emode_for_corrupt_record(), so we should do the same here. This part of the patch is perhaps a bug fix, but I don't think it has any important consequences, so no back-patch. The point here is just to continue to whittle down the entirely excessive use of global variables in xlog.c. Discussion:

Heikki Linnakangas pushed:

Andres Freund pushed:

Daniel Gustafsson pushed: