PostgreSQL Weekly News - August 22, 2021

Posted on 2021-08-23 by PWN

PostgreSQL Weekly News - August 22, 2021

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Applied Patches

Michaël Paquier pushed:

  • Refresh apply delay on reload of recovery_min_apply_delay at recovery. This commit ensures that the wait interval in the replay delay loop waiting for an amount of time defined by recovery_min_apply_delay is correctly handled on reload, recalculating the delay if this GUC value is updated, based on the timestamp of the commit record being replayed. The previous behavior would be problematic for example with replay still waiting even if the delay got reduced or just cancelled. If the apply delay was increased to a larger value, the wait would have just respected the old value set, finishing earlier. Author: Soumyadeep Chakraborty, Ashwin Agrawal Reviewed-by: Kyotaro Horiguchi, Michael Paquier Discussion: Backpatch-through: 9.6

  • Revert refactoring of hex code to src/common/. This is a combined revert of the following commits: - c3826f8, a refactoring piece that moved the hex decoding code to src/common/. This code was cleaned up by aef8948, as it originally included no overflow checks in the same way as the base64 routines in src/common/ used by SCRAM, making it unsafe for its purpose. - aef8948, a more advanced refactoring of the hex encoding/decoding code to src/common/ that added sanity checks on the result buffer for hex decoding and encoding. As reported by Hans Buschmann, those overflow checks are expensive, and it is possible to see a performance drop in the decoding/encoding of bytea or LOs the longer they are. Simple SQLs working on large bytea values show a clear difference in perf profile. - ccf4e27, a cleanup made possible by aef8948. The reverts of all those commits bring back the performance of hex decoding and encoding back to what it was in ~13. Fow now and post-beta3, this is the simplest option. Reported-by: Hans Buschmann Discussion: Backpatch-through: 14

  • Improve performance of float overflow checks in btree_gist. The current code could do unnecessary calls to isinf() (two for the argument values all the time while one could be sufficient in some cases). zero_is_valid was never used but the result value was still checked on 0 in the first position of the check. This is similar to 607f8ce. btree_gist has just copy-pasted the code doing those checks from the backend float4/8 code, as of the macro CHECKFLOATVAL(), to do the work. Author: Haiying Tang Discussion:

Daniel Gustafsson pushed:

John Naylor pushed:

Tom Lane pushed:

  • Reduce memory consumption for pending invalidation messages. The existing data structures in inval.c are fairly inefficient for the common case of a command or subtransaction that registers a small number of cache invalidation events. While this doesn't matter if we commit right away, it can build up to a lot of bloat in a transaction that contains many DDL operations. By making a few more assumptions about the expected use-case, we can switch to a representation using densely-packed arrays. Although this eliminates some data-copying, it doesn't seem to make much difference time-wise. But the space consumption decreases substantially. Patch by me; thanks to Nathan Bossart for review. Discussion:

  • Improve regex compiler's arc moving/copying logic. The functions moveins(), copyins(), moveouts(), copyouts() are required to preserve the invariant that there are no duplicate arcs in the regex's NFA. Spencer's original implementation of them was O(N^2) since it checked separately for a match to each source arc. In commit 579840ca0 I improved that by adding sort/merge logic to be used if more than a few arcs are to be moved/copied. However, I now realize that that missed a bet. At many call sites, the target state is newly made and cannot have any existing in-arcs (respectively out-arcs) that could be duplicates. So spending any cycles at all on checking for duplicates is wasted effort; in these cases we can just blindly move/copy all the source arcs. Add code paths to do that. It turns out that for copyins()/copyouts(), all the call sites have this property, making all the "improved" logic in them flat out unreachable. Perhaps we'll need the full capability again someday, so I just #ifdef'd those paths out rather than removing them entirely. In passing, add a few test cases to improve code coverage in this area as well as in regc_locale.c/regc_pg_locale.c. Discussion:

  • Reduce assumptions about locale's behavior in new regex tests. I was overoptimistic to assume that UTF8-based locales would all consider U+1500 to be a member of the [[:alpha:]] char class. Tweak the test cases added by commit 78a843f11 to avoid that assumption. We might need to lobotomize them further, but this should be enough to fix the early buildfarm reports.

  • Prevent ALTER TYPE/DOMAIN/OPERATOR from changing extension membership. If recordDependencyOnCurrentExtension is invoked on a pre-existing, free-standing object during an extension update script, that object will become owned by the extension. In our current code this is possible in three cases: * Replacing a "shell" type or operator. * CREATE OR REPLACE overwriting an existing object. * ALTER TYPE SET, ALTER DOMAIN SET, and ALTER OPERATOR SET. The first of these cases is intentional behavior, as noted by the existing comments for GenerateTypeDependencies. It seems like appropriate behavior for CREATE OR REPLACE too; at least, the obvious alternatives are not better. However, the fact that it happens during ALTER is an artifact of trying to share code (GenerateTypeDependencies and makeOperatorDependencies) between the CREATE and ALTER cases. Since an extension script would be unlikely to ALTER an object that didn't already belong to the extension, this behavior is not very troubling for the direct target object ... but ALTER TYPE SET will recurse to dependent domains, and it is very uncool for those to become owned by the extension if they were not already. Let's fix this by redefining the ALTER cases to never change extension membership, full stop. We could minimize the behavioral change by only changing the behavior when ALTER TYPE SET is recursing to a domain, but that would complicate the code and it does not seem like a better definition. Per bug #17144 from Alex Kozhemyakin. Back-patch to v13 where ALTER TYPE SET was added. (The other cases are older, but since they only affect the directly-named object, there's not enough of a problem to justify changing the behavior further back.) Discussion:

  • Fix check_agg_arguments' examination of aggregate FILTER clauses. Recursion into the FILTER clause was mis-implemented, such that a relevant Var or Aggref at the very top of the FILTER clause would be ignored. (Of course, that'd have to be a plain boolean Var or boolean-returning aggregate.) The consequence would be mis-identification of the correct semantic level of the aggregate, which could lead to not-per-spec query behavior. If the FILTER expression is an aggregate, this could also lead to failure to issue an expected "aggregate function calls cannot be nested" error, which would likely result in a core dump later on, since the planner and executor aren't expecting such cases to appear. The root cause is that commit b560ec1b0 blindly copied some code that assumed it's recursing into a List, and thus didn't examine the top-level node. To forestall questions about why this call doesn't look like the others, as well as possible future copy-and-paste mistakes, let's change all three check_agg_arguments_walker calls in check_agg_arguments, even though only the one for the filter clause is really broken. Per bug #17152 from Zhiyong Wu. This has been wrong since we implemented FILTER, so back-patch to all supported versions. (Testing suggests that pre-v11 branches manage to avoid crashing in the bad-Aggref case, thanks to "redundant" checks in ExecInitAgg. But I'm not sure how thorough that protection is, and anyway the wrong-behavior issue remains, so fix 9.6 and 10 too.) Discussion:

  • Avoid trying to lock OLD/NEW in a rule with FOR UPDATE. transformLockingClause neglected to exclude the pseudo-RTEs for OLD/NEW when processing a rule's query. This led to odd errors or even crashes later on. This bug is very ancient, but it's not terribly surprising that nobody noticed, since the use-case for SELECT FOR UPDATE in a non-view rule is somewhere between thin and non-existent. Still, crashing is not OK. Per bug #17151 from Zhiyong Wu. Thanks to Masahiko Sawada for analysis of the problem. Discussion:

  • Fix performance bug in regexp's citerdissect/creviterdissect. After detecting a sub-match "dissect" failure (i.e., a backref match failure) in the i'th sub-match of an iteration node, we should proceed by adjusting the attempted length of the i'th submatch. As coded, though, these functions changed the attempted length of the last sub-match, and only after exhausting all possibilities for that would they back up to adjust the next-to-last sub-match, and then the second-from-last, etc; all of which is wasted effort, since only changing the start or length of the i'th sub-match can possibly make it succeed. This oversight creates the possibility for exponentially bad performance. Fortunately the problem is masked in most cases by optimizations or constraints applied elsewhere; which explains why we'd not noticed it before. But it is possible to reach the problem with fairly simple, if contrived, regexps. Oversight in my commit 173e29aa5. That's pretty ancient now, so back-patch to all supported branches. Discussion:

  • Improve error messages about misuse of SELECT INTO. Improve two places in plpgsql, and one in spi.c, where an error message would confusingly tell you that you couldn't use a SELECT query, when what you had written was a SELECT query. The actual problem is that you can't use SELECT ... INTO in these contexts, but the messages failed to make that apparent. Special-case SELECT INTO to make these errors more helpful. Also, fix the same spots in plpgsql, as well as several messages in exec_eval_expr(), to not quote the entire complained-of query or expression in the primary error message. That behavior very easily led to violating our message style guideline about keeping the primary error message short and single-line. Also, since the important part of the message was after the inserted text, it could make the real problem very hard to see. We can report the query or expression as the first line of errcontext instead. Per complaint from Roger Mason. Back-patch to v14, since (a) some of these messages are new in v14 and (b) v14's translatable strings are still somewhat in flux. The problem's older than that of course, but I'm hesitant to change the behavior further back. Discussion:

Álvaro Herrera pushed:

  • Revert analyze support for partitioned tables. This reverts the following commits: 1b5617eb844cd2470a334c1d2eec66cf9b39c41a Describe (auto-)analyze behavior for partitioned tables 0e69f705cc1a3df273b38c9883fb5765991e04fe Set pg_class.reltuples for partitioned tables 41badeaba8beee7648ebe7923a41c04f1f3cb302 Document ANALYZE storage parameters for partitioned tables 0827e8af70f4653ba17ed773f123a60eadd9f9c9 autovacuum: handle analyze for partitioned tables There are efficiency issues in this code when handling databases with large numbers of partitions, and it doesn't look like there isn't any trivial way to handle those. There are some other issues as well. It's now too late in the cycle for nontrivial fixes, so we'll have to let Postgres 14 users continue to manually deal with ANALYZE their partitioned tables, and hopefully we can fix the issues for Postgres 15. I kept [most of] be280cdad298 ("Don't reset relhasindex for partitioned tables on ANALYZE") because while we added it due to 0827e8af70f4, it is a good bugfix in its own right, since it affects manual analyze as well as autovacuum-induced analyze, and there's no reason to revert it. I retained the addition of relkind 'p' to tables included by pg_stat_user_tables, because reverting that would require a catversion bump. Also, in pg14 only, I keep a struct member that was added to PgStat_TabStatEntry to avoid breaking compatibility with existing stat files. Backpatch to 14. Discussion:

Heikki Linnakangas pushed:

Michael Meskes pushed:

Amit Kapila pushed:

Andres Freund pushed:

  • Unset MyBEEntry, making elog.c's call to pgstat_get_my_query_id() safe. Previously log messages late during shutdown could end up using either another backend's PgBackendStatus (multi user) or segfault (single user) because pgstat_get_my_query_id()'s check for !MyBEEntry didn't filter out use after pgstat_beshutdown_hook(). This became a bug in 4f0b0966c86, but was a bit fishy before. But given there's no known problematic cases before 14, it doesn't seem worth backpatching further. Also fixes a wrong filename in a comment, introduced in e1025044. Reported-By: Andres Freund Reviewed-By: Julien Rouhaud Discussion: Rouhaud Backpatch: 14-

Peter Eisentraut pushed:

David Rowley pushed: