diff options
Diffstat (limited to 'src/include/access/htup.h')
| -rw-r--r-- | src/include/access/htup.h | 41 |
1 files changed, 12 insertions, 29 deletions
diff --git a/src/include/access/htup.h b/src/include/access/htup.h index 37ba1164ec1..971a279b9c9 100644 --- a/src/include/access/htup.h +++ b/src/include/access/htup.h @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.67 2004/07/11 18:01:45 tgl Exp $ + * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.68 2004/08/01 17:32:19 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -68,34 +68,17 @@ * object ID (if HEAP_HASOID is set in t_infomask) * user data fields * - * We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac - * in just three physical fields. Xmin is always really stored, but - * Cmin and Xmax share a field, as do Cmax and Xvac. This works because - * we know that there are only a limited number of states that a tuple can - * be in, and that Cmin and Cmax are only interesting for the lifetime of - * the inserting and deleting transactions respectively. We have the - * following possible states of a tuple: + * We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in four + * physical fields. Xmin, Cmin and Xmax are always really stored, but + * Cmax and Xvac share a field. This works because we know that there are + * only a limited number of states that a tuple can be in, and that Cmax + * is only interesting for the lifetime of the deleting transaction. + * This assumes that VACUUM FULL never tries to move a tuple whose Cmax + * is still interesting (ie, delete-in-progress). * - * XMIN CMIN XMAX CMAX XVAC - * - * NEW (never deleted, not moved by vacuum): - * valid valid invalid invalid invalid - * - * DELETED BY CREATING XACT: - * valid valid = XMIN valid invalid - * - * DELETED BY OTHER XACT: - * valid unneeded valid valid invalid - * - * MOVED BY VACUUM FULL: - * valid unneeded maybe-valid unneeded valid - * - * This assumes that VACUUM FULL never tries to move a tuple whose Cmin or - * Cmax is still interesting (ie, insert-in-progress or delete-in-progress). - * - * This table shows that if we use an infomask bit to handle the case - * XMAX=XMIN specially, we never need to store Cmin and Xmax at the same - * time. Nor do we need to store Cmax and Xvac at the same time. + * Note that in 7.3 and 7.4 a similar idea was applied to Xmax and Cmin. + * However, with the advent of subtransactions, a tuple may need both Xmax + * and Cmin simultaneously, so this is no longer possible. * * Following the fixed header fields, the nulls bitmap is stored (beginning * at t_bits). The bitmap is *not* stored if t_infomask shows that there @@ -416,7 +399,7 @@ typedef HeapTupleData *HeapTuple; * WAL record definitions for heapam.c's WAL operations * * XLOG allows to store some information in high 4 bits of log - * record xl_info field + * record xl_info field. We use 3 for opcode and one for init bit. */ #define XLOG_HEAP_INSERT 0x00 #define XLOG_HEAP_DELETE 0x10 |