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Should collation change invalidate index? #10

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dwsteele opened this issue Feb 1, 2017 · 0 comments
Open

Should collation change invalidate index? #10

dwsteele opened this issue Feb 1, 2017 · 0 comments
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dwsteele commented Feb 1, 2017

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@dwsteele dwsteele self-assigned this Feb 1, 2017
dwsteele pushed a commit that referenced this issue Mar 29, 2019
This adds a flag "deterministic" to collations.  If that is false,
such a collation disables various optimizations that assume that
strings are equal only if they are byte-wise equal.  That then allows
use cases such as case-insensitive or accent-insensitive comparisons
or handling of strings with different Unicode normal forms.

This functionality is only supported with the ICU provider.  At least
glibc doesn't appear to have any locales that work in a
nondeterministic way, so it's not worth supporting this for the libc
provider.

The term "deterministic comparison" in this context is from Unicode
Technical Standard #10
(https://unicode.org/reports/tr10/#Deterministic_Comparison).

This patch makes changes in three areas:

- CREATE COLLATION DDL changes and system catalog changes to support
  this new flag.

- Many executor nodes and auxiliary code are extended to track
  collations.  Previously, this code would just throw away collation
  information, because the eventually-called user-defined functions
  didn't use it since they only cared about equality, which didn't
  need collation information.

- String data type functions that do equality comparisons and hashing
  are changed to take the (non-)deterministic flag into account.  For
  comparison, this just means skipping various shortcuts and tie
  breakers that use byte-wise comparison.  For hashing, we first need
  to convert the input string to a canonical "sort key" using the ICU
  analogue of strxfrm().

Reviewed-by: Daniel Verite <[email protected]>
Reviewed-by: Peter Geoghegan <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/[email protected]
dwsteele pushed a commit that referenced this issue Oct 2, 2024
1. TruncateMultiXact() performs the SLRU truncations in a critical
section. Deleting the SLRU segments calls ForwardSyncRequest(), which
will try to compact the request queue if it's full
(CompactCheckpointerRequestQueue()). That in turn allocates memory,
which is not allowed in a critical section. Backtrace:

    TRAP: failed Assert("CritSectionCount == 0 || (context)->allowInCritSection"), File: "../src/backend/utils/mmgr/mcxt.c", Line: 1353, PID: 920981
    postgres: autovacuum worker template0(ExceptionalCondition+0x6e)[0x560a501e866e]
    postgres: autovacuum worker template0(+0x5dce3d)[0x560a50217e3d]
    postgres: autovacuum worker template0(ForwardSyncRequest+0x8e)[0x560a4ffec95e]
    postgres: autovacuum worker template0(RegisterSyncRequest+0x2b)[0x560a50091eeb]
    postgres: autovacuum worker template0(+0x187b0a)[0x560a4fdc2b0a]
    postgres: autovacuum worker template0(SlruDeleteSegment+0x101)[0x560a4fdc2ab1]
    postgres: autovacuum worker template0(TruncateMultiXact+0x2fb)[0x560a4fdbde1b]
    postgres: autovacuum worker template0(vac_update_datfrozenxid+0x4b3)[0x560a4febd2f3]
    postgres: autovacuum worker template0(+0x3adf66)[0x560a4ffe8f66]
    postgres: autovacuum worker template0(AutoVacWorkerMain+0x3ed)[0x560a4ffe7c2d]
    postgres: autovacuum worker template0(+0x3b1ead)[0x560a4ffecead]
    postgres: autovacuum worker template0(+0x3b620e)[0x560a4fff120e]
    postgres: autovacuum worker template0(+0x3b3fbb)[0x560a4ffeefbb]
    postgres: autovacuum worker template0(+0x2f724e)[0x560a4ff3224e]
    /lib/x86_64-linux-gnu/libc.so.6(+0x27c8a)[0x7f62cc642c8a]
    /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f62cc642d45]
    postgres: autovacuum worker template0(_start+0x21)[0x560a4fd16f31]

To fix, bail out in CompactCheckpointerRequestQueue() without doing
anything, if it's called in a critical section. That covers the above
call path, as well as any other similar cases where
RegisterSyncRequest might be called in a critical section.

2. After fixing that, another problem became apparent: Autovacuum
process doing that truncation can deadlock with the checkpointer
process. TruncateMultiXact() sets "MyProc->delayChkptFlags |=
DELAY_CHKPT_START". If the sync request queue is full and cannot be
compacted, the process will repeatedly sleep and retry, until there is
room in the queue. However, if the checkpointer is trying to start a
checkpoint at the same time, and is waiting for the DELAY_CHKPT_START
processes to finish, the queue will never shrink.

More concretely, the autovacuum process is stuck here:

    #0  0x00007fc934926dc3 in epoll_wait () from /lib/x86_64-linux-gnu/libc.so.6
    #1  0x000056220b24348b in WaitEventSetWaitBlock (set=0x56220c2e4b50, occurred_events=0x7ffe7856d040, nevents=1, cur_timeout=<optimized out>) at ../src/backend/storage/ipc/latch.c:1570
    #2  WaitEventSetWait (set=0x56220c2e4b50, timeout=timeout@entry=10, occurred_events=<optimized out>, occurred_events@entry=0x7ffe7856d040, nevents=nevents@entry=1,
        wait_event_info=wait_event_info@entry=150994949) at ../src/backend/storage/ipc/latch.c:1516
    #3  0x000056220b243224 in WaitLatch (latch=<optimized out>, latch@entry=0x0, wakeEvents=wakeEvents@entry=40, timeout=timeout@entry=10, wait_event_info=wait_event_info@entry=150994949)
        at ../src/backend/storage/ipc/latch.c:538
    #4  0x000056220b26cf46 in RegisterSyncRequest (ftag=ftag@entry=0x7ffe7856d0a0, type=type@entry=SYNC_FORGET_REQUEST, retryOnError=true) at ../src/backend/storage/sync/sync.c:614
    #5  0x000056220af9db0a in SlruInternalDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1495
    #6  0x000056220af9dab1 in SlruDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1566
    #7  0x000056220af98e1b in PerformMembersTruncation (oldestOffset=<optimized out>, newOldestOffset=<optimized out>) at ../src/backend/access/transam/multixact.c:3006
    #8  TruncateMultiXact (newOldestMulti=newOldestMulti@entry=3221225472, newOldestMultiDB=newOldestMultiDB@entry=4) at ../src/backend/access/transam/multixact.c:3201
    #9  0x000056220b098303 in vac_truncate_clog (frozenXID=749, minMulti=<optimized out>, lastSaneFrozenXid=749, lastSaneMinMulti=3221225472) at ../src/backend/commands/vacuum.c:1917
    #10 vac_update_datfrozenxid () at ../src/backend/commands/vacuum.c:1760
    #11 0x000056220b1c3f76 in do_autovacuum () at ../src/backend/postmaster/autovacuum.c:2550
    #12 0x000056220b1c2c3d in AutoVacWorkerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/autovacuum.c:1569

and the checkpointer is stuck here:

    #0  0x00007fc9348ebf93 in clock_nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
    #1  0x00007fc9348fe353 in nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
    #2  0x000056220b40ecb4 in pg_usleep (microsec=microsec@entry=10000) at ../src/port/pgsleep.c:50
    #3  0x000056220afb43c3 in CreateCheckPoint (flags=flags@entry=108) at ../src/backend/access/transam/xlog.c:7098
    #4  0x000056220b1c6e86 in CheckpointerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/checkpointer.c:464

To fix, add AbsorbSyncRequests() to the loops where the checkpointer
waits for DELAY_CHKPT_START or DELAY_CHKPT_COMPLETE operations to
finish.

Backpatch to v14. Before that, SLRU deletion didn't call
RegisterSyncRequest, which avoided this failure. I'm not sure if there
are other similar scenarios on older versions, but we haven't had
any such reports.

Discussion: https://www.postgresql.org/message-id/[email protected]
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