*** a/src/backend/access/nbtree/nbtsearch.c
--- b/src/backend/access/nbtree/nbtsearch.c
***************
*** 849,856 **** _bt_first(IndexScanDesc scan, ScanDirection dir)
  	if (!BufferIsValid(buf))
  	{
  		/*
! 		 * We only get here if the index is completely empty.
! 		 * Lock relation because nothing finer to lock exists.
  		 */
  		PredicateLockRelation(rel, scan->xs_snapshot);
  		return false;
--- 849,856 ----
  	if (!BufferIsValid(buf))
  	{
  		/*
! 		 * We only get here if the index is completely empty. Lock relation
! 		 * because nothing finer to lock exists.
  		 */
  		PredicateLockRelation(rel, scan->xs_snapshot);
  		return false;
*** a/src/backend/access/transam/twophase_rmgr.c
--- b/src/backend/access/transam/twophase_rmgr.c
***************
*** 26,32 **** const TwoPhaseCallback twophase_recover_callbacks[TWOPHASE_RM_MAX_ID + 1] =
  	NULL,						/* END ID */
  	lock_twophase_recover,		/* Lock */
  	NULL,						/* pgstat */
! 	multixact_twophase_recover,	/* MultiXact */
  	predicatelock_twophase_recover		/* PredicateLock */
  };
  
--- 26,32 ----
  	NULL,						/* END ID */
  	lock_twophase_recover,		/* Lock */
  	NULL,						/* pgstat */
! 	multixact_twophase_recover, /* MultiXact */
  	predicatelock_twophase_recover		/* PredicateLock */
  };
  
***************
*** 44,50 **** const TwoPhaseCallback twophase_postabort_callbacks[TWOPHASE_RM_MAX_ID + 1] =
  	NULL,						/* END ID */
  	lock_twophase_postabort,	/* Lock */
  	pgstat_twophase_postabort,	/* pgstat */
! 	multixact_twophase_postabort,	/* MultiXact */
  	NULL						/* PredicateLock */
  };
  
--- 44,50 ----
  	NULL,						/* END ID */
  	lock_twophase_postabort,	/* Lock */
  	pgstat_twophase_postabort,	/* pgstat */
! 	multixact_twophase_postabort,		/* MultiXact */
  	NULL						/* PredicateLock */
  };
  
*** a/src/backend/storage/lmgr/predicate.c
--- b/src/backend/storage/lmgr/predicate.c
***************
*** 483,492 **** SerializationNeededForRead(Relation relation, Snapshot snapshot)
  	 * MySerializableXact, so that subsequent calls to this function can exit
  	 * quickly.
  	 *
! 	 * A transaction is flagged as RO_SAFE if all concurrent R/W
! 	 * transactions commit without having conflicts out to an earlier
! 	 * snapshot, thus ensuring that no conflicts are possible for this
! 	 * transaction.
  	 */
  	if (SxactIsROSafe(MySerializableXact))
  	{
--- 483,491 ----
  	 * MySerializableXact, so that subsequent calls to this function can exit
  	 * quickly.
  	 *
! 	 * A transaction is flagged as RO_SAFE if all concurrent R/W transactions
! 	 * commit without having conflicts out to an earlier snapshot, thus
! 	 * ensuring that no conflicts are possible for this transaction.
  	 */
  	if (SxactIsROSafe(MySerializableXact))
  	{
***************
*** 498,504 **** SerializationNeededForRead(Relation relation, Snapshot snapshot)
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
! 	return true;	/* no excuse to skip predicate locking */
  }
  
  /*
--- 497,503 ----
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
! 	return true;				/* no excuse to skip predicate locking */
  }
  
  /*
***************
*** 516,522 **** SerializationNeededForWrite(Relation relation)
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
! 	return true;	/* no excuse to skip predicate locking */
  }
  
  
--- 515,521 ----
  	if (!PredicateLockingNeededForRelation(relation))
  		return false;
  
! 	return true;				/* no excuse to skip predicate locking */
  }
  
  
*** a/src/include/storage/predicate.h
--- b/src/include/storage/predicate.h
***************
*** 54,60 **** 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, const Snapshot snapshot);
  extern void CheckForSerializableConflictIn(const Relation relation, const HeapTuple tuple, const Buffer buffer);
  extern void CheckTableForSerializableConflictIn(const Relation relation);
  
--- 54,60 ----
  
  /* conflict detection (may also trigger rollback) */
  extern void CheckForSerializableConflictOut(const bool valid, const Relation relation, const HeapTuple tuple,
! 								const Buffer buffer, const Snapshot snapshot);
  extern void CheckForSerializableConflictIn(const Relation relation, const HeapTuple tuple, const Buffer buffer);
  extern void CheckTableForSerializableConflictIn(const Relation relation);
  
*** a/src/include/storage/predicate_internals.h
--- b/src/include/storage/predicate_internals.h
***************
*** 90,98 **** typedef struct SERIALIZABLEXACT
  	int			pid;			/* pid of associated process */
  } SERIALIZABLEXACT;
  
! #define SXACT_FLAG_COMMITTED				0x00000001 /* already committed */
! #define SXACT_FLAG_PREPARED					0x00000002 /* about to commit */
! #define SXACT_FLAG_DOOMED					0x00000004 /* will roll back */
  /*
   * The following flag actually means that the flagged transaction has a
   * conflict out *to a transaction which committed ahead of it*.  It's hard
--- 90,98 ----
  	int			pid;			/* pid of associated process */
  } SERIALIZABLEXACT;
  
! #define SXACT_FLAG_COMMITTED				0x00000001	/* already committed */
! #define SXACT_FLAG_PREPARED					0x00000002	/* about to commit */
! #define SXACT_FLAG_DOOMED					0x00000004	/* will roll back */
  /*
   * The following flag actually means that the flagged transaction has a
   * conflict out *to a transaction which committed ahead of it*.  It's hard
***************
*** 132,139 **** typedef struct PredXactListData
  	/*
  	 * These global variables are maintained when registering and cleaning up
  	 * serializable transactions.  They must be global across all backends,
! 	 * but are not needed outside the predicate.c source file. Protected
! 	 * by SerializableXactHashLock.
  	 */
  	TransactionId SxactGlobalXmin;		/* global xmin for active serializable
  										 * transactions */
--- 132,139 ----
  	/*
  	 * These global variables are maintained when registering and cleaning up
  	 * serializable transactions.  They must be global across all backends,
! 	 * but are not needed outside the predicate.c source file. Protected by
! 	 * SerializableXactHashLock.
  	 */
  	TransactionId SxactGlobalXmin;		/* global xmin for active serializable
  										 * transactions */