diff options
Diffstat (limited to 'src/include/utils/snapshot.h')
| -rw-r--r-- | src/include/utils/snapshot.h | 32 |
1 files changed, 30 insertions, 2 deletions
diff --git a/src/include/utils/snapshot.h b/src/include/utils/snapshot.h index c542238825a..4b256074b0b 100644 --- a/src/include/utils/snapshot.h +++ b/src/include/utils/snapshot.h @@ -30,6 +30,22 @@ typedef struct SnapshotData *Snapshot; typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup, Snapshot snapshot, Buffer buffer); +/* + * Struct representing all kind of possible snapshots. + * + * There are several different kinds of snapshots: + * * Normal MVCC snapshots + * * MVCC snapshots taken during recovery (in Hot-Standby mode) + * * Historic MVCC snapshots used during logical decoding + * * snapshots passed to HeapTupleSatisfiesDirty() + * * snapshots used for SatisfiesAny, Toast, Self where no members are + * accessed. + * + * TODO: It's probably a good idea to split this struct using a NodeTag + * similar to how parser and executor nodes are handled, with one type for + * each different kind of snapshot to avoid overloading the meaning of + * individual fields. + */ typedef struct SnapshotData { SnapshotSatisfiesFunc satisfies; /* tuple test function */ @@ -46,11 +62,23 @@ typedef struct SnapshotData */ TransactionId xmin; /* all XID < xmin are visible to me */ TransactionId xmax; /* all XID >= xmax are invisible to me */ - TransactionId *xip; /* array of xact IDs in progress */ + /* + * For normal MVCC snapshot this contains the all xact IDs that are in + * progress, unless the snapshot was taken during recovery in which case + * it's empty. For historic MVCC snapshots, the meaning is inverted, + * i.e. it contains *committed* transactions between xmin and xmax. + */ + TransactionId *xip; uint32 xcnt; /* # of xact ids in xip[] */ /* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */ int32 subxcnt; /* # of xact ids in subxip[] */ - TransactionId *subxip; /* array of subxact IDs in progress */ + /* + * For non-historic MVCC snapshots, this contains subxact IDs that are in + * progress (and other transactions that are in progress if taken during + * recovery). For historic snapshot it contains *all* xids assigned to the + * replayed transaction, including the toplevel xid. + */ + TransactionId *subxip; bool suboverflowed; /* has the subxip array overflowed? */ bool takenDuringRecovery; /* recovery-shaped snapshot? */ bool copied; /* false if it's a static snapshot */ |