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
bb16aba50c Enable parallel query with SERIALIZABLE isolation.
doc/src/sgml/monitoring.sgml | 7 +-
doc/src/sgml/parallel.sgml | 17 --
src/backend/access/nbtree/nbtsort.c | 2 +-
src/backend/access/transam/parallel.c | 18 +-
src/backend/access/transam/xact.c | 7 +-
src/backend/executor/execParallel.c | 2 +-
src/backend/optimizer/plan/planner.c | 11 +-
src/backend/storage/lmgr/lwlock.c | 1 +
src/backend/storage/lmgr/predicate.c | 237 +++++++++++++++++++--
src/backend/utils/resowner/resowner.c | 2 +-
src/include/access/parallel.h | 3 +-
src/include/storage/lwlock.h | 1 +
src/include/storage/predicate.h | 11 +-
src/include/storage/predicate_internals.h | 10 +
.../isolation/expected/serializable-parallel-2.out | 44 ++++
.../isolation/expected/serializable-parallel.out | 44 ++++
src/test/isolation/isolation_schedule | 2 +
.../isolation/specs/serializable-parallel-2.spec | 30 +++
.../isolation/specs/serializable-parallel.spec | 47 ++++
19 files changed, 429 insertions(+), 67 deletions(-)