dafaa3efb7
Until now, our Serializable mode has in fact been what's called Snapshot Isolation, which allows some anomalies that could not occur in any serialized ordering of the transactions. This patch fixes that using a method called Serializable Snapshot Isolation, based on research papers by Michael J. Cahill (see README-SSI for full references). In Serializable Snapshot Isolation, transactions run like they do in Snapshot Isolation, but a predicate lock manager observes the reads and writes performed and aborts transactions if it detects that an anomaly might occur. This method produces some false positives, ie. it sometimes aborts transactions even though there is no anomaly. To track reads we implement predicate locking, see storage/lmgr/predicate.c. Whenever a tuple is read, a predicate lock is acquired on the tuple. Shared memory is finite, so when a transaction takes many tuple-level locks on a page, the locks are promoted to a single page-level lock, and further to a single relation level lock if necessary. To lock key values with no matching tuple, a sequential scan always takes a relation-level lock, and an index scan acquires a page-level lock that covers the search key, whether or not there are any matching keys at the moment. A predicate lock doesn't conflict with any regular locks or with another predicate locks in the normal sense. They're only used by the predicate lock manager to detect the danger of anomalies. Only serializable transactions participate in predicate locking, so there should be no extra overhead for for other transactions. Predicate locks can't be released at commit, but must be remembered until all the transactions that overlapped with it have completed. That means that we need to remember an unbounded amount of predicate locks, so we apply a lossy but conservative method of tracking locks for committed transactions. If we run short of shared memory, we overflow to a new "pg_serial" SLRU pool. We don't currently allow Serializable transactions in Hot Standby mode. That would be hard, because even read-only transactions can cause anomalies that wouldn't otherwise occur. Serializable isolation mode now means the new fully serializable level. Repeatable Read gives you the old Snapshot Isolation level that we have always had. Kevin Grittner and Dan Ports, reviewed by Jeff Davis, Heikki Linnakangas and Anssi Kääriäinen
68 lines
2.5 KiB
C
68 lines
2.5 KiB
C
/*-------------------------------------------------------------------------
|
|
*
|
|
* predicate.h
|
|
* POSTGRES public predicate locking definitions.
|
|
*
|
|
*
|
|
* Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
|
|
* Portions Copyright (c) 1994, Regents of the University of California
|
|
*
|
|
* src/include/storage/predicate.h
|
|
*
|
|
*-------------------------------------------------------------------------
|
|
*/
|
|
#ifndef PREDICATE_H
|
|
#define PREDICATE_H
|
|
|
|
#include "utils/relcache.h"
|
|
#include "utils/snapshot.h"
|
|
|
|
|
|
/*
|
|
* GUC variables
|
|
*/
|
|
extern int max_predicate_locks_per_xact;
|
|
|
|
|
|
/* Number of SLRU buffers to use for predicate locking */
|
|
#define NUM_OLDSERXID_BUFFERS 16
|
|
|
|
|
|
/*
|
|
* function prototypes
|
|
*/
|
|
|
|
/* housekeeping for shared memory predicate lock structures */
|
|
extern void InitPredicateLocks(void);
|
|
extern Size PredicateLockShmemSize(void);
|
|
|
|
/* predicate lock reporting */
|
|
extern bool PageIsPredicateLocked(const Relation relation, const BlockNumber blkno);
|
|
|
|
/* predicate lock maintenance */
|
|
extern Snapshot RegisterSerializableTransaction(Snapshot snapshot);
|
|
extern void RegisterPredicateLockingXid(const TransactionId xid);
|
|
extern void PredicateLockRelation(const Relation relation);
|
|
extern void PredicateLockPage(const Relation relation, const BlockNumber blkno);
|
|
extern void PredicateLockTuple(const Relation relation, const HeapTuple tuple);
|
|
extern void PredicateLockTupleRowVersionLink(const Relation relation, const HeapTuple oldTuple, const HeapTuple newTuple);
|
|
extern void PredicateLockPageSplit(const Relation relation, const BlockNumber oldblkno, const BlockNumber newblkno);
|
|
extern void PredicateLockPageCombine(const Relation relation, const BlockNumber oldblkno, const BlockNumber newblkno);
|
|
extern void ReleasePredicateLocks(const bool isCommit);
|
|
|
|
/* conflict detection (may also trigger rollback) */
|
|
extern void CheckForSerializableConflictOut(const bool valid, const Relation relation, const HeapTuple tuple, const Buffer buffer);
|
|
extern void CheckForSerializableConflictIn(const Relation relation, const HeapTuple tuple, const Buffer buffer);
|
|
|
|
/* final rollback checking */
|
|
extern void PreCommit_CheckForSerializationFailure(void);
|
|
|
|
/* two-phase commit support */
|
|
extern void AtPrepare_PredicateLocks(void);
|
|
extern void PostPrepare_PredicateLocks(TransactionId xid);
|
|
extern void PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
|
|
extern void predicatelock_twophase_recover(TransactionId xid, uint16 info,
|
|
void *recdata, uint32 len);
|
|
|
|
#endif /* PREDICATE_H */
|