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Combine cmin and cmax fields of HeapTupleHeaders into a single field, by
keeping private state in each backend that has inserted and deleted the same tuple during its current top-level transaction. This is sufficient since there is no need to be able to determine the cmin/cmax from any other transaction. This gets us back down to 23-byte headers, removing a penalty paid in 8.0 to support subtransactions. Patch by Heikki Linnakangas, with minor revisions by moi, following a design hashed out awhile back on the pghackers list.
This commit is contained in:
Tom Lane
@@ -7,7 +7,7 @@
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* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/include/access/htup.h,v 1.90 2007/02/05 04:22:18 tgl Exp $
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* $PostgreSQL: pgsql/src/include/access/htup.h,v 1.91 2007/02/09 03:35:34 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -45,7 +45,7 @@
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/*
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* Heap tuple header. To avoid wasting space, the fields should be
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* layed out in such a way to avoid structure padding.
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* laid out in such a way as to avoid structure padding.
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*
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* Datums of composite types (row types) share the same general structure
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* as on-disk tuples, so that the same routines can be used to build and
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@@ -65,17 +65,18 @@
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* object ID (if HEAP_HASOID is set in t_infomask)
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* user data fields
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*
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* We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in four
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* physical fields. Xmin, Cmin and Xmax are always really stored, but
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* Cmax and Xvac share a field. This works because we know that there are
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* only a limited number of states that a tuple can be in, and that Cmax
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* is only interesting for the lifetime of the deleting transaction.
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* This assumes that VACUUM FULL never tries to move a tuple whose Cmax
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* is still interesting (ie, delete-in-progress).
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*
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* Note that in 7.3 and 7.4 a similar idea was applied to Xmax and Cmin.
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* However, with the advent of subtransactions, a tuple may need both Xmax
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* and Cmin simultaneously, so this is no longer possible.
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* We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in three
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* physical fields. Xmin and Xmax are always really stored, but Cmin, Cmax
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* and Xvac share a field. This works because we know that Cmin and Cmax
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* are only interesting for the lifetime of the inserting and deleting
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* transaction respectively. If a tuple is inserted and deleted in the same
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* transaction, we store a "combo" command id that can be mapped to the real
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* cmin and cmax, but only by use of local state within the originating
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* backend. See combocid.c for more details. Meanwhile, Xvac is only set
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* by VACUUM FULL, which does not have any command sub-structure and so does
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* not need either Cmin or Cmax. (This requires that VACUUM FULL never try
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* to move a tuple whose Cmin or Cmax is still interesting, ie, an insert-
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* in-progress or delete-in-progress tuple.)
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*
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* A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
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* is initialized with its own TID (location). If the tuple is ever updated,
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@@ -103,14 +104,13 @@
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typedef struct HeapTupleFields
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{
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TransactionId t_xmin; /* inserting xact ID */
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CommandId t_cmin; /* inserting command ID */
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TransactionId t_xmax; /* deleting or locking xact ID */
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union
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{
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CommandId t_cmax; /* deleting or locking command ID */
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CommandId t_cid; /* inserting or deleting command ID, or both */
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TransactionId t_xvac; /* VACUUM FULL xact ID */
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} t_field4;
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} t_field3;
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} HeapTupleFields;
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typedef struct DatumTupleFields
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@@ -145,7 +145,7 @@ typedef struct HeapTupleHeaderData
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uint8 t_hoff; /* sizeof header incl. bitmap, padding */
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/* ^ - 27 bytes - ^ */
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/* ^ - 23 bytes - ^ */
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bits8 t_bits[1]; /* bitmap of NULLs -- VARIABLE LENGTH */
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@@ -163,7 +163,7 @@ typedef HeapTupleHeaderData *HeapTupleHeader;
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#define HEAP_HASCOMPRESSED 0x0008 /* has compressed stored attribute(s) */
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#define HEAP_HASEXTENDED 0x000C /* the two above combined */
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#define HEAP_HASOID 0x0010 /* has an object-id field */
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/* 0x0020 is presently unused */
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#define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */
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#define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
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#define HEAP_XMAX_SHARED_LOCK 0x0080 /* xmax is shared locker */
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/* if either LOCK bit is set, xmax hasn't deleted the tuple, only locked it */
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@@ -180,17 +180,13 @@ typedef HeapTupleHeaderData *HeapTupleHeader;
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* FULL */
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#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)
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#define HEAP_XACT_MASK 0xFFC0 /* visibility-related bits */
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#define HEAP_XACT_MASK 0xFFE0 /* visibility-related bits */
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/* information stored in t_infomask2, and accessor macros */
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/*
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* information stored in t_infomask2:
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*/
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#define HEAP_NATTS_MASK 0x7FF /* 11 bits for number of attributes */
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/* bits 0xF800 are unused */
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#define HeapTupleHeaderGetNatts(tup) ((tup)->t_infomask2 & HEAP_NATTS_MASK)
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#define HeapTupleHeaderSetNatts(tup, natts) \
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( \
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(tup)->t_infomask2 = ((tup)->t_infomask2 & ~HEAP_NATTS_MASK) | (natts) \
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)
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/* bits 0xF800 are currently unused */
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/*
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* HeapTupleHeader accessor macros
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@@ -219,39 +215,40 @@ typedef HeapTupleHeaderData *HeapTupleHeader;
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TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_xmax) \
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)
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#define HeapTupleHeaderGetCmin(tup) \
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( \
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(tup)->t_choice.t_heap.t_cmin \
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)
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#define HeapTupleHeaderSetCmin(tup, cid) \
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( \
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(tup)->t_choice.t_heap.t_cmin = (cid) \
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)
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/*
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* Note: GetCmax will produce wrong answers after SetXvac has been executed
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* by a transaction other than the inserting one. We could check
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* HEAP_XMAX_INVALID and return FirstCommandId if it's clear, but since that
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* bit will be set again if the deleting transaction aborts, there'd be no
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* real gain in safety from the extra test. So, just rely on the caller not
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* to trust the value unless it's meaningful.
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* HeapTupleHeaderGetRawCommandId will give you what's in the header whether
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* it is useful or not. Most code should use HeapTupleHeaderGetCmin or
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* HeapTupleHeaderGetCmax instead, but note that those Assert that you can
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* get a legitimate result, ie you are in the originating transaction!
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*/
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#define HeapTupleHeaderGetCmax(tup) \
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#define HeapTupleHeaderGetRawCommandId(tup) \
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( \
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(tup)->t_choice.t_heap.t_field4.t_cmax \
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(tup)->t_choice.t_heap.t_field3.t_cid \
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)
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#define HeapTupleHeaderSetCmax(tup, cid) \
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/* SetCmin is reasonably simple since we never need a combo CID */
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#define HeapTupleHeaderSetCmin(tup, cid) \
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do { \
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Assert(!((tup)->t_infomask & HEAP_MOVED)); \
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(tup)->t_choice.t_heap.t_field4.t_cmax = (cid); \
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(tup)->t_choice.t_heap.t_field3.t_cid = (cid); \
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(tup)->t_infomask &= ~HEAP_COMBOCID; \
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} while (0)
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/* SetCmax must be used after HeapTupleHeaderAdjustCmax; see combocid.c */
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#define HeapTupleHeaderSetCmax(tup, cid, iscombo) \
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do { \
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Assert(!((tup)->t_infomask & HEAP_MOVED)); \
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(tup)->t_choice.t_heap.t_field3.t_cid = (cid); \
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if (iscombo) \
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(tup)->t_infomask |= HEAP_COMBOCID; \
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else \
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(tup)->t_infomask &= ~HEAP_COMBOCID; \
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} while (0)
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#define HeapTupleHeaderGetXvac(tup) \
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( \
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((tup)->t_infomask & HEAP_MOVED) ? \
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(tup)->t_choice.t_heap.t_field4.t_xvac \
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(tup)->t_choice.t_heap.t_field3.t_xvac \
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: \
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InvalidTransactionId \
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)
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@@ -259,7 +256,7 @@ do { \
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#define HeapTupleHeaderSetXvac(tup, xid) \
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do { \
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Assert((tup)->t_infomask & HEAP_MOVED); \
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TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_field4.t_xvac); \
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TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_field3.t_xvac); \
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} while (0)
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#define HeapTupleHeaderGetDatumLength(tup) \
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@@ -306,6 +303,14 @@ do { \
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*((Oid *) ((char *)(tup) + (tup)->t_hoff - sizeof(Oid))) = (oid); \
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} while (0)
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#define HeapTupleHeaderGetNatts(tup) \
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((tup)->t_infomask2 & HEAP_NATTS_MASK)
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#define HeapTupleHeaderSetNatts(tup, natts) \
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( \
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(tup)->t_infomask2 = ((tup)->t_infomask2 & ~HEAP_NATTS_MASK) | (natts) \
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)
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/*
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* BITMAPLEN(NATTS) -
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@@ -374,9 +379,9 @@ do { \
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* and thereby prevent accidental use of the nonexistent fields.
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*
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* MinimalTupleData contains a length word, some padding, and fields matching
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* HeapTupleHeaderData beginning with t_infomask2. The padding is chosen so that
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* offsetof(t_infomask2) is the same modulo MAXIMUM_ALIGNOF in both structs.
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* This makes data alignment rules equivalent in both cases.
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* HeapTupleHeaderData beginning with t_infomask2. The padding is chosen so
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* that offsetof(t_infomask2) is the same modulo MAXIMUM_ALIGNOF in both
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* structs. This makes data alignment rules equivalent in both cases.
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*
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* When a minimal tuple is accessed via a HeapTupleData pointer, t_data is
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* set to point MINIMAL_TUPLE_OFFSET bytes before the actual start of the
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@@ -405,7 +410,7 @@ typedef struct MinimalTupleData
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uint8 t_hoff; /* sizeof header incl. bitmap, padding */
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/* ^ - 27 bytes - ^ */
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/* ^ - 23 bytes - ^ */
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bits8 t_bits[1]; /* bitmap of NULLs -- VARIABLE LENGTH */
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@@ -638,4 +643,11 @@ typedef struct xl_heap_freeze
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#define SizeOfHeapFreeze (offsetof(xl_heap_freeze, cutoff_xid) + sizeof(TransactionId))
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/* HeapTupleHeader functions implemented in utils/time/combocid.c */
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extern CommandId HeapTupleHeaderGetCmin(HeapTupleHeader tup);
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extern CommandId HeapTupleHeaderGetCmax(HeapTupleHeader tup);
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extern void HeapTupleHeaderAdjustCmax(HeapTupleHeader tup,
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CommandId *cmax,
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bool *iscombo);
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#endif /* HTUP_H */
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