@@ -50,13 +50,15 @@ typedef enum SnapshotType
5050SNAPSHOT_MVCC = 0 ,
5151
5252/*-------------------------------------------------------------------------
53- * A tuple is visible iff the tuple is valid including effects of open
54- * transactions.
53+ * A tuple is visible iff the tuple is valid "for itself".
5554 *
5655 * Here, we consider the effects of:
57- * - all committedand in-progress transactions (as of the current instant)
56+ * - all committed transactions (as of the current instant)
5857 * - previous commands of this transaction
5958 * - changes made by the current command
59+ *
60+ * Does _not_ include:
61+ * - in-progress transactions (as of the current instant)
6062 * -------------------------------------------------------------------------
6163 */
6264SNAPSHOT_SELF ,
@@ -79,6 +81,22 @@ typedef enum SnapshotType
7981 * - all committed and in-progress transactions (as of the current instant)
8082 * - previous commands of this transaction
8183 * - changes made by the current command
84+ *
85+ * This is essentially like SNAPSHOT_SELF as far as effects of the current
86+ * transaction and committed/aborted xacts are concerned. However, it
87+ * also includes the effects of other xacts still in progress.
88+ *
89+ * A special hack is that when a snapshot of this type is used to
90+ * determine tuple visibility, the passed-in snapshot struct is used as an
91+ * output argument to return the xids of concurrent xacts that affected
92+ * the tuple. snapshot->xmin is set to the tuple's xmin if that is
93+ * another transaction that's still in progress; or to
94+ * InvalidTransactionId if the tuple's xmin is committed good, committed
95+ * dead, or my own xact. Similarly for snapshot->xmax and the tuple's
96+ * xmax. If the tuple was inserted speculatively, meaning that the
97+ * inserter might still back down on the insertion without aborting the
98+ * whole transaction, the associated token is also returned in
99+ * snapshot->speculativeToken. See also InitDirtySnapshot().
82100 * -------------------------------------------------------------------------
83101 */
84102SNAPSHOT_DIRTY ,
@@ -92,9 +110,10 @@ typedef enum SnapshotType
92110
93111/*
94112 * A tuple is visible iff the tuple might be visible to some transaction;
95- * false if it's surely dead to everyone,ie , vacuumable.
113+ * false if it's surely dead to everyone,i.e. , vacuumable.
96114 *
97- * Snapshot.xmin must have been set up with the xmin horizon to use.
115+ * For visibility checks snapshot->min must have been set up with the xmin
116+ * horizon to use.
98117 */
99118SNAPSHOT_NON_VACUUMABLE
100119}SnapshotType ;