From 2d871385cdecd2b860f5eeb2615d8fcf9e866e54 Mon Sep 17 00:00:00 2001 From: Michael Paquier Date: Thu, 17 Jan 2019 11:54:42 +0900 Subject: [PATCH 1/3] Refactor code to wait for older snapshots into its own routine This is being used by CREATE INDEX CONCURRENTLY to make sure that valid indexes are marked as such after waiting for all transactions using snapshots older than the reference snapshot used for concurrent index validation are gone. This piece is useful independently, and can be used by REINDEX CONCURRENTLY. --- src/backend/commands/indexcmds.c | 71 +--------------------------- src/backend/utils/time/snapmgr.c | 81 ++++++++++++++++++++++++++++++++ src/include/utils/snapmgr.h | 2 + 3 files changed, 85 insertions(+), 69 deletions(-) diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c index 1959e8a82e..a81e656059 100644 --- a/src/backend/commands/indexcmds.c +++ b/src/backend/commands/indexcmds.c @@ -362,9 +362,7 @@ DefineIndex(Oid relationId, int numberOfAttributes; int numberOfKeyAttributes; TransactionId limitXmin; - VirtualTransactionId *old_snapshots; ObjectAddress address; - int n_old_snapshots; LockRelId heaprelid; LOCKTAG heaplocktag; LOCKMODE lockmode; @@ -1252,74 +1250,9 @@ DefineIndex(Oid relationId, * The index is now valid in the sense that it contains all currently * interesting tuples. But since it might not contain tuples deleted just * before the reference snap was taken, we have to wait out any - * transactions that might have older snapshots. Obtain a list of VXIDs - * of such transactions, and wait for them individually. - * - * We can exclude any running transactions that have xmin > the xmin of - * our reference snapshot; their oldest snapshot must be newer than ours. - * We can also exclude any transactions that have xmin = zero, since they - * evidently have no live snapshot at all (and any one they might be in - * process of taking is certainly newer than ours). Transactions in other - * DBs can be ignored too, since they'll never even be able to see this - * index. - * - * We can also exclude autovacuum processes and processes running manual - * lazy VACUUMs, because they won't be fazed by missing index entries - * either. (Manual ANALYZEs, however, can't be excluded because they - * might be within transactions that are going to do arbitrary operations - * later.) - * - * Also, GetCurrentVirtualXIDs never reports our own vxid, so we need not - * check for that. - * - * If a process goes idle-in-transaction with xmin zero, we do not need to - * wait for it anymore, per the above argument. We do not have the - * infrastructure right now to stop waiting if that happens, but we can at - * least avoid the folly of waiting when it is idle at the time we would - * begin to wait. We do this by repeatedly rechecking the output of - * GetCurrentVirtualXIDs. If, during any iteration, a particular vxid - * doesn't show up in the output, we know we can forget about it. + * transactions that might have older snapshots. */ - old_snapshots = GetCurrentVirtualXIDs(limitXmin, true, false, - PROC_IS_AUTOVACUUM | PROC_IN_VACUUM, - &n_old_snapshots); - - for (i = 0; i < n_old_snapshots; i++) - { - if (!VirtualTransactionIdIsValid(old_snapshots[i])) - continue; /* found uninteresting in previous cycle */ - - if (i > 0) - { - /* see if anything's changed ... */ - VirtualTransactionId *newer_snapshots; - int n_newer_snapshots; - int j; - int k; - - newer_snapshots = GetCurrentVirtualXIDs(limitXmin, - true, false, - PROC_IS_AUTOVACUUM | PROC_IN_VACUUM, - &n_newer_snapshots); - for (j = i; j < n_old_snapshots; j++) - { - if (!VirtualTransactionIdIsValid(old_snapshots[j])) - continue; /* found uninteresting in previous cycle */ - for (k = 0; k < n_newer_snapshots; k++) - { - if (VirtualTransactionIdEquals(old_snapshots[j], - newer_snapshots[k])) - break; - } - if (k >= n_newer_snapshots) /* not there anymore */ - SetInvalidVirtualTransactionId(old_snapshots[j]); - } - pfree(newer_snapshots); - } - - if (VirtualTransactionIdIsValid(old_snapshots[i])) - VirtualXactLock(old_snapshots[i], true); - } + WaitForOlderSnapshots(limitXmin); /* * Index can now be marked valid -- update its pg_index entry diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index f93b37b9c9..1dc3c162db 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -1990,6 +1990,87 @@ MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin) LWLockRelease(OldSnapshotTimeMapLock); } +/* + * WaitForOlderSnapshots + * + * Wait for transactions that might have an older snapshot than the given xmin + * limit, because it might not contain tuples deleted just before it has + * been taken. Obtain a list of VXIDs of such transactions, and wait for them + * individually. + * + * We can exclude any running transactions that have xmin > the xmin given; + * their oldest snapshot must be newer than our xmin limit. + * We can also exclude any transactions that have xmin = zero, since they + * evidently have no live snapshot at all (and any one they might be in + * process of taking is certainly newer than ours). Transactions in other + * DBs can be ignored too, since they'll never even be able to see this + * index. + * + * We can also exclude autovacuum processes and processes running manual + * lazy VACUUMs, because they won't be fazed by missing index entries + * either. (Manual ANALYZEs, however, can't be excluded because they + * might be within transactions that are going to do arbitrary operations + * later.) + * + * Also, GetCurrentVirtualXIDs never reports our own vxid, so we need not + * check for that. + * + * If a process goes idle-in-transaction with xmin zero, we do not need to + * wait for it anymore, per the above argument. We do not have the + * infrastructure right now to stop waiting if that happens, but we can at + * least avoid the folly of waiting when it is idle at the time we would + * begin to wait. We do this by repeatedly rechecking the output of + * GetCurrentVirtualXIDs. If, during any iteration, a particular vxid + * doesn't show up in the output, we know we can forget about it. + */ +void +WaitForOlderSnapshots(TransactionId limitXmin) +{ + int i, + n_old_snapshots; + VirtualTransactionId *old_snapshots; + + old_snapshots = GetCurrentVirtualXIDs(limitXmin, true, false, + PROC_IS_AUTOVACUUM | PROC_IN_VACUUM, + &n_old_snapshots); + + for (i = 0; i < n_old_snapshots; i++) + { + if (!VirtualTransactionIdIsValid(old_snapshots[i])) + continue; /* found uninteresting in previous cycle */ + + if (i > 0) + { + /* see if anything's changed ... */ + VirtualTransactionId *newer_snapshots; + int n_newer_snapshots; + int j; + int k; + + newer_snapshots = GetCurrentVirtualXIDs(limitXmin, + true, false, + PROC_IS_AUTOVACUUM | PROC_IN_VACUUM, + &n_newer_snapshots); + for (j = i; j < n_old_snapshots; j++) + { + if (!VirtualTransactionIdIsValid(old_snapshots[j])) + continue; /* found uninteresting in previous cycle */ + for (k = 0; k < n_newer_snapshots; k++) + { + if (VirtualTransactionIdEquals(old_snapshots[j], + newer_snapshots[k])) + break; + } + if (k >= n_newer_snapshots) /* not there anymore */ + SetInvalidVirtualTransactionId(old_snapshots[j]); + } + pfree(newer_snapshots); + } + + if (VirtualTransactionIdIsValid(old_snapshots[i])) + VirtualXactLock(old_snapshots[i], true); + } +} /* * Setup a snapshot that replaces normal catalog snapshots that allows catalog diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index f8308e6925..41c6b908d6 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -98,6 +98,8 @@ extern void MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, extern char *ExportSnapshot(Snapshot snapshot); +extern void WaitForOlderSnapshots(TransactionId limitXmin); + /* Support for catalog timetravel for logical decoding */ struct HTAB; extern struct HTAB *HistoricSnapshotGetTupleCids(void); -- 2.20.1