[PATCH 3/5] Move conversion of a "historic" to MVCC snapshot to a separate function.
Antonin Houska <ah@cybertec.at>
From: Antonin Houska <ah@cybertec.at>
To:
Date: 2025-12-09T18:44:42Z
Lists: pgsql-hackers
The conversion is now handled by SnapBuildMVCCFromHistoric(). REPACK
CONCURRENTLY will also need it.
---
src/backend/commands/cluster.c | 8 ++-
src/backend/replication/logical/snapbuild.c | 57 +++++++++++++++++----
src/backend/utils/time/snapmgr.c | 3 +-
src/include/replication/snapbuild.h | 1 +
src/include/utils/snapmgr.h | 1 +
5 files changed, 53 insertions(+), 17 deletions(-)
diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 3afab656cd9..89f0b03a31c 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -70,8 +70,7 @@ typedef struct
static bool cluster_rel_recheck(RepackCommand cmd, Relation OldHeap,
Oid indexOid, Oid userid, int options);
-static void rebuild_relation(RepackCommand cmd,
- Relation OldHeap, Relation index, bool verbose);
+static void rebuild_relation(Relation OldHeap, Relation index, bool verbose);
static void copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex,
bool verbose, bool *pSwapToastByContent,
TransactionId *pFreezeXid, MultiXactId *pCutoffMulti);
@@ -415,7 +414,7 @@ cluster_rel(RepackCommand cmd, Relation OldHeap, Oid indexOid,
TransferPredicateLocksToHeapRelation(OldHeap);
/* rebuild_relation does all the dirty work */
- rebuild_relation(cmd, OldHeap, index, verbose);
+ rebuild_relation(OldHeap, index, verbose);
/* rebuild_relation closes OldHeap, and index if valid */
out:
@@ -629,8 +628,7 @@ mark_index_clustered(Relation rel, Oid indexOid, bool is_internal)
* On exit, they are closed, but locks on them are not released.
*/
static void
-rebuild_relation(RepackCommand cmd,
- Relation OldHeap, Relation index, bool verbose)
+rebuild_relation(Relation OldHeap, Relation index, bool verbose)
{
Oid tableOid = RelationGetRelid(OldHeap);
Oid accessMethod = OldHeap->rd_rel->relam;
diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 6e18baa33cb..34bdd987478 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -440,10 +440,7 @@ Snapshot
SnapBuildInitialSnapshot(SnapBuild *builder)
{
Snapshot snap;
- TransactionId xid;
TransactionId safeXid;
- TransactionId *newxip;
- int newxcnt = 0;
Assert(XactIsoLevel == XACT_REPEATABLE_READ);
Assert(builder->building_full_snapshot);
@@ -485,7 +482,33 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
MyProc->xmin = snap->xmin;
- /* allocate in transaction context */
+ /* Convert the historic snapshot to MVCC snapshot. */
+ return SnapBuildMVCCFromHistoric(snap, true);
+}
+
+/*
+ * Turn a historic MVCC snapshot into an ordinary MVCC snapshot.
+ *
+ * Unlike a regular (non-historic) MVCC snapshot, the 'xip' array of this
+ * snapshot contains not only running main transactions, but also their
+ * subtransactions. On the other hand, 'subxip' will usually be empty. This
+ * difference does not affect the result of XidInMVCCSnapshot() because it
+ * searches both in 'xip' and 'subxip'.
+ *
+ * Pass true for 'in_place' if you don't care about modifying the source
+ * snapshot. If you need a new instance, and one that was allocated as a
+ * single chunk of memory, pass false.
+ */
+Snapshot
+SnapBuildMVCCFromHistoric(Snapshot snapshot, bool in_place)
+{
+ TransactionId xid;
+ TransactionId *oldxip = snapshot->xip;
+ uint32 oldxcnt = snapshot->xcnt;
+ TransactionId *newxip;
+ int newxcnt = 0;
+ Snapshot result;
+
newxip = (TransactionId *)
palloc(sizeof(TransactionId) * GetMaxSnapshotXidCount());
@@ -495,7 +518,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
* classical snapshot by marking all non-committed transactions as
* in-progress. This can be expensive.
*/
- for (xid = snap->xmin; NormalTransactionIdPrecedes(xid, snap->xmax);)
+ for (xid = snapshot->xmin; NormalTransactionIdPrecedes(xid, snapshot->xmax);)
{
void *test;
@@ -503,7 +526,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
* Check whether transaction committed using the decoding snapshot
* meaning of ->xip.
*/
- test = bsearch(&xid, snap->xip, snap->xcnt,
+ test = bsearch(&xid, snapshot->xip, snapshot->xcnt,
sizeof(TransactionId), xidComparator);
if (test == NULL)
@@ -520,11 +543,25 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
}
/* adjust remaining snapshot fields as needed */
- snap->snapshot_type = SNAPSHOT_MVCC;
- snap->xcnt = newxcnt;
- snap->xip = newxip;
+ snapshot->xcnt = newxcnt;
+ snapshot->xip = newxip;
+
+ if (in_place)
+ result = snapshot;
+ else
+ {
+ result = CopySnapshot(snapshot);
+
+ /* Restore the original values so the source is intact. */
+ snapshot->xip = oldxip;
+ snapshot->xcnt = oldxcnt;
+
+ /* newxip has been copied */
+ pfree(newxip);
+ }
+ result->snapshot_type = SNAPSHOT_MVCC;
- return snap;
+ return result;
}
/*
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 24f73a49d27..886060305f5 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -213,7 +213,6 @@ typedef struct ExportedSnapshot
static List *exportedSnapshots = NIL;
/* Prototypes for local functions */
-static Snapshot CopySnapshot(Snapshot snapshot);
static void UnregisterSnapshotNoOwner(Snapshot snapshot);
static void FreeSnapshot(Snapshot snapshot);
static void SnapshotResetXmin(void);
@@ -604,7 +603,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
* The copy is palloc'd in TopTransactionContext and has initial refcounts set
* to 0. The returned snapshot has the copied flag set.
*/
-static Snapshot
+Snapshot
CopySnapshot(Snapshot snapshot)
{
Snapshot newsnap;
diff --git a/src/include/replication/snapbuild.h b/src/include/replication/snapbuild.h
index 44031dcf6e3..6d4d2d1814c 100644
--- a/src/include/replication/snapbuild.h
+++ b/src/include/replication/snapbuild.h
@@ -73,6 +73,7 @@ extern void FreeSnapshotBuilder(SnapBuild *builder);
extern void SnapBuildSnapDecRefcount(Snapshot snap);
extern Snapshot SnapBuildInitialSnapshot(SnapBuild *builder);
+extern Snapshot SnapBuildMVCCFromHistoric(Snapshot snapshot, bool in_place);
extern const char *SnapBuildExportSnapshot(SnapBuild *builder);
extern void SnapBuildClearExportedSnapshot(void);
extern void SnapBuildResetExportedSnapshotState(void);
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 604c1f90216..f65f83c85cd 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -63,6 +63,7 @@ extern Snapshot GetTransactionSnapshot(void);
extern Snapshot GetLatestSnapshot(void);
extern void SnapshotSetCommandId(CommandId curcid);
+extern Snapshot CopySnapshot(Snapshot snapshot);
extern Snapshot GetCatalogSnapshot(Oid relid);
extern Snapshot GetNonHistoricCatalogSnapshot(Oid relid);
extern void InvalidateCatalogSnapshot(void);
--
2.47.3
--=-=-=
Content-Type: text/plain
Content-Disposition: attachment;
filename=v27-0004-Add-CONCURRENTLY-option-to-REPACK-command.patch