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Enable parallel query with SERIALIZABLE isolation.

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Previously, the SERIALIZABLE isolation level prevented parallel query
from being used.  Allow the two features to be used together by
sharing the leader's SERIALIZABLEXACT with parallel workers.

An extra per-SERIALIZABLEXACT LWLock is introduced to make it safe to
share, and new logic is introduced to coordinate the early release
of the SERIALIZABLEXACT required for the SXACT_FLAG_RO_SAFE
optimization, as follows:

The first backend to observe the SXACT_FLAG_RO_SAFE flag (set by
some other transaction) will 'partially release' the SERIALIZABLEXACT,
meaning that the conflicts and locks it holds are released, but the
SERIALIZABLEXACT itself will remain active because other backends
might still have a pointer to it.

Whenever any backend notices the SXACT_FLAG_RO_SAFE flag, it clears
its own MySerializableXact variable and frees local resources so that
it can skip SSI checks for the rest of the transaction.  In the
special case of the leader process, it transfers the SERIALIZABLEXACT
to a new variable SavedSerializableXact, so that it can be completely
released at the end of the transaction after all workers have exited.

Remove the serializable_okay flag added to CreateParallelContext() by
commit 9da0cc35, because it's now redundant.

Author: Thomas Munro
Reviewed-by: Haribabu Kommi, Robert Haas, Masahiko Sawada, Kevin Grittner
Discussion: https://postgr.es/m/CAEepm=0gXGYhtrVDWOTHS8SQQy_=S9xo+8oCxGLWZAOoeJ=yzQ@mail.gmail.com
This commit is contained in:
Thomas Munro
2019-03-15 16:23:46 +13:00
parent 13e8643bfc
commit bb16aba50c
19 changed files with 429 additions and 67 deletions
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11
src/include/storage/predicate.h
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@@ -30,6 +30,11 @@ extern int max_predicate_locks_per_page;
/* Number of SLRU buffers to use for predicate locking */
#define NUM_OLDSERXID_BUFFERS 16
/*
* A handle used for sharing SERIALIZABLEXACT objects between the participants
* in a parallel query.
*/
typedef void *SerializableXactHandle;
/*
* function prototypes
@@ -56,7 +61,7 @@ extern void PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snap
extern void PredicateLockPageSplit(Relation relation, BlockNumber oldblkno, BlockNumber newblkno);
extern void PredicateLockPageCombine(Relation relation, BlockNumber oldblkno, BlockNumber newblkno);
extern void TransferPredicateLocksToHeapRelation(Relation relation);
extern void ReleasePredicateLocks(bool isCommit);
extern void ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe);
/* conflict detection (may also trigger rollback) */
extern void CheckForSerializableConflictOut(bool valid, Relation relation, HeapTuple tuple,
@@ -74,4 +79,8 @@ extern void PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
extern void predicatelock_twophase_recover(TransactionId xid, uint16 info,
void *recdata, uint32 len);
/* parallel query support */
extern SerializableXactHandle ShareSerializableXact(void);
extern void AttachSerializableXact(SerializableXactHandle handle);
#endif /* PREDICATE_H */