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Commit90c885c

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Increment xactCompletionCount during subtransaction abort.
Snapshot caching, introduced in623a9ba, did not incrementxactCompletionCount during subtransaction abort. That could lead to an oldersnapshot being reused. That is, at least as far as I can see, not acorrectness issue (for MVCC snapshots there's no difference between "inprogress" and "aborted"). The only difference between the old and newsnapshots would be a newer ->xmax.While HeapTupleSatisfiesMVCC makes the same visibility determination, reusingthe old snapshot leads HeapTupleSatisfiesMVCC to not setHEAP_XMIN_INVALID. Which subsequently causes the kill_prior_tuple optimizationto not kick in (via HeapTupleIsSurelyDead() returning false). The performanceeffects of doing the same index-lookups over and over again is how the issuewas discovered...Fix the issue by incrementing xactCompletionCount inXidCacheRemoveRunningXids. It already acquires ProcArrayLock exclusively,making that an easy proposition.Add a test to ensure that kill_prior_tuple prevents index growth when itinvolves aborted subtransaction of the current transaction.Author: Andres FreundDiscussion:https://postgr.es/m/20210406043521.lopeo7bbigad3n6t@alap3.anarazel.deDiscussion:https://postgr.es/m/20210317055718.v6qs3ltzrformqoa%40alap3.anarazel.de
1 parent8523492 commit90c885c

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5 files changed

+96
-1
lines changed

5 files changed

+96
-1
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‎src/backend/storage/ipc/procarray.c

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Original file line numberDiff line numberDiff line change
@@ -1210,6 +1210,11 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
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*/
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MaintainLatestCompletedXidRecovery(running->latestCompletedXid);
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/*
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* NB: No need to increment ShmemVariableCache->xactCompletionCount here,
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* nobody can see it yet.
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*/
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LWLockRelease(ProcArrayLock);
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/* ShmemVariableCache->nextXid must be beyond any observed xid. */
@@ -3915,6 +3920,9 @@ XidCacheRemoveRunningXids(TransactionId xid,
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/* Also advance global latestCompletedXid while holding the lock */
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MaintainLatestCompletedXid(latestXid);
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/* ... and xactCompletionCount */
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ShmemVariableCache->xactCompletionCount++;
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LWLockRelease(ProcArrayLock);
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}
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‎src/test/regress/expected/mvcc.out

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@@ -0,0 +1,42 @@
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--
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-- Verify that index scans encountering dead rows produced by an
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-- aborted subtransaction of the current transaction can utilize the
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-- kill_prio_tuple optimization
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--
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-- NB: The table size is currently *not* expected to stay the same, we
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-- don't have logic to trigger opportunistic pruning in cases like
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-- this.
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BEGIN;
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SET LOCAL enable_seqscan = false;
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SET LOCAL enable_indexonlyscan = false;
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SET LOCAL enable_bitmapscan = false;
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-- Can't easily use a unique index, since dead tuples can be found
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-- independent of the kill_prior_tuples optimization.
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CREATE TABLE clean_aborted_self(key int, data text);
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CREATE INDEX clean_aborted_self_key ON clean_aborted_self(key);
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INSERT INTO clean_aborted_self (key, data) VALUES (-1, 'just to allocate metapage');
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-- save index size from before the changes, for comparison
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SELECT pg_relation_size('clean_aborted_self_key') AS clean_aborted_self_key_before \gset
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DO $$
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BEGIN
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-- iterate often enough to see index growth even on larger-than-default page sizes
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FOR i IN 1..100 LOOP
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BEGIN
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-- perform index scan over all the inserted keys to get them to be seen as dead
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IF EXISTS(SELECT * FROM clean_aborted_self WHERE key > 0 AND key < 100) THEN
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RAISE data_corrupted USING MESSAGE = 'these rows should not exist';
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END IF;
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INSERT INTO clean_aborted_self SELECT g.i, 'rolling back in a sec' FROM generate_series(1, 100) g(i);
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-- just some error that's not normally thrown
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RAISE reading_sql_data_not_permitted USING MESSAGE = 'round and round again';
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EXCEPTION WHEN reading_sql_data_not_permitted THEN END;
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END LOOP;
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END;$$;
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-- show sizes only if they differ
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SELECT :clean_aborted_self_key_before AS size_before, pg_relation_size('clean_aborted_self_key') size_after
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WHERE :clean_aborted_self_key_before != pg_relation_size('clean_aborted_self_key');
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size_before | size_after
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-------------+------------
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(0 rows)
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ROLLBACK;

‎src/test/regress/parallel_schedule

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@@ -29,7 +29,7 @@ test: strings numerology point lseg line box path polygon circle date time timet
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# geometry depends on point, lseg, box, path, polygon and circle
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# horology depends on interval, timetz, timestamp, timestamptz
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# ----------
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test: geometry horology regex type_sanity opr_sanity misc_sanity comments expressions unicode xid
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test: geometry horology regex type_sanity opr_sanity misc_sanity comments expressions unicode xid mvcc
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# ----------
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# These four each depend on the previous one

‎src/test/regress/serial_schedule

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@@ -11,6 +11,7 @@ test: int4
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test: int8
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test: oid
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test: xid
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test: mvcc
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test: float4
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test: float8
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test: bit

‎src/test/regress/sql/mvcc.sql

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--
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-- Verify that index scans encountering dead rows produced by an
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-- aborted subtransaction of the current transaction can utilize the
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-- kill_prio_tuple optimization
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--
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-- NB: The table size is currently *not* expected to stay the same, we
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-- don't have logic to trigger opportunistic pruning in cases like
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-- this.
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BEGIN;
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SET LOCAL enable_seqscan= false;
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SET LOCAL enable_indexonlyscan= false;
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SET LOCAL enable_bitmapscan= false;
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-- Can't easily use a unique index, since dead tuples can be found
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-- independent of the kill_prior_tuples optimization.
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CREATETABLEclean_aborted_self(keyint, datatext);
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CREATEINDEXclean_aborted_self_keyON clean_aborted_self(key);
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INSERT INTO clean_aborted_self (key, data)VALUES (-1,'just to allocate metapage');
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-- save index size from before the changes, for comparison
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SELECT pg_relation_size('clean_aborted_self_key')AS clean_aborted_self_key_before \gset
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DO $$
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BEGIN
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-- iterate often enough to see index growth even on larger-than-default page sizes
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FOR iIN1..100 LOOP
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BEGIN
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-- perform index scan over all the inserted keys to get them to be seen as dead
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IF EXISTS(SELECT*FROM clean_aborted_selfWHERE key>0AND key<100) THEN
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RAISE data_corrupted USING MESSAGE='these rows should not exist';
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END IF;
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INSERT INTO clean_aborted_selfSELECTg.i,'rolling back in a sec'FROM generate_series(1,100) g(i);
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-- just some error that's not normally thrown
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RAISE reading_sql_data_not_permitted USING MESSAGE='round and round again';
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EXCEPTION WHEN reading_sql_data_not_permitted THEN END;
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END LOOP;
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END;$$;
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-- show sizes only if they differ
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SELECT :clean_aborted_self_key_beforeAS size_before, pg_relation_size('clean_aborted_self_key') size_after
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WHERE :clean_aborted_self_key_before!= pg_relation_size('clean_aborted_self_key');
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ROLLBACK;

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