diff options
Diffstat (limited to 'src/include/access/htup.h')
| -rw-r--r-- | src/include/access/htup.h | 71 |
1 files changed, 66 insertions, 5 deletions
diff --git a/src/include/access/htup.h b/src/include/access/htup.h index 57db9f84934..170ad657fb7 100644 --- a/src/include/access/htup.h +++ b/src/include/access/htup.h @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.82 2006/05/10 23:18:39 tgl Exp $ + * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.83 2006/06/27 02:51:39 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -31,7 +31,7 @@ */ #define MaxTupleAttributeNumber 1664 /* 8 * 208 */ -/*---------- +/* * MaxHeapAttributeNumber limits the number of (user) columns in a table. * This should be somewhat less than MaxTupleAttributeNumber. It must be * at least one less, else we will fail to do UPDATEs on a maximal-width @@ -42,11 +42,10 @@ * In any case, depending on column data types you will likely be running * into the disk-block-based limit on overall tuple size if you have more * than a thousand or so columns. TOAST won't help. - *---------- */ #define MaxHeapAttributeNumber 1600 /* 8 * 200 */ -/*---------- +/* * Heap tuple header. To avoid wasting space, the fields should be * layed out in such a way to avoid structure padding. * @@ -101,7 +100,6 @@ * t_infomask), then it is stored just before the user data, which begins at * the offset shown by t_hoff. Note that t_hoff must be a multiple of * MAXALIGN. - *---------- */ typedef struct HeapTupleFields @@ -141,6 +139,8 @@ typedef struct HeapTupleHeaderData ItemPointerData t_ctid; /* current TID of this or newer tuple */ + /* Fields below here must match MinimalTupleData! */ + int16 t_natts; /* number of attributes */ uint16 t_infomask; /* various flag bits, see below */ @@ -355,6 +355,62 @@ do { \ /* + * MinimalTuple is an alternate representation that is used for transient + * tuples inside the executor, in places where transaction status information + * is not required, the tuple rowtype is known, and shaving off a few bytes + * is worthwhile because we need to store many tuples. The representation + * is chosen so that tuple access routines can work with either full or + * minimal tuples via a HeapTupleData pointer structure. The access routines + * see no difference, except that they must not access the transaction status + * or t_ctid fields because those aren't there. + * + * For the most part, MinimalTuples should be accessed via TupleTableSlot + * routines. These routines will prevent access to the "system columns" + * and thereby prevent accidental use of the nonexistent fields. + * + * MinimalTupleData contains a length word, some padding, and fields matching + * HeapTupleHeaderData beginning with t_natts. The padding is chosen so that + * offsetof(t_natts) is the same modulo MAXIMUM_ALIGNOF in both structs. + * This makes data alignment rules equivalent in both cases. + * + * When a minimal tuple is accessed via a HeapTupleData pointer, t_data is + * set to point MINIMAL_TUPLE_OFFSET bytes before the actual start of the + * minimal tuple --- that is, where a full tuple matching the minimal tuple's + * data would start. This trick is what makes the structs seem equivalent. + * + * Note that t_hoff is computed the same as in a full tuple, hence it includes + * the MINIMAL_TUPLE_OFFSET distance. t_len does not include that, however. + */ +#define MINIMAL_TUPLE_OFFSET \ + ((offsetof(HeapTupleHeaderData, t_natts) - sizeof(uint32)) / MAXIMUM_ALIGNOF * MAXIMUM_ALIGNOF) +#define MINIMAL_TUPLE_PADDING \ + ((offsetof(HeapTupleHeaderData, t_natts) - sizeof(uint32)) % MAXIMUM_ALIGNOF) + +typedef struct MinimalTupleData +{ + uint32 t_len; /* actual length of minimal tuple */ + + char mt_padding[MINIMAL_TUPLE_PADDING]; + + /* Fields below here must match HeapTupleHeaderData! */ + + int16 t_natts; /* number of attributes */ + + uint16 t_infomask; /* various flag bits, see below */ + + uint8 t_hoff; /* sizeof header incl. bitmap, padding */ + + /* ^ - 27 bytes - ^ */ + + bits8 t_bits[1]; /* bitmap of NULLs -- VARIABLE LENGTH */ + + /* MORE DATA FOLLOWS AT END OF STRUCT */ +} MinimalTupleData; + +typedef MinimalTupleData *MinimalTuple; + + +/* * HeapTupleData is an in-memory data structure that points to a tuple. * * There are several ways in which this data structure is used: @@ -376,6 +432,11 @@ do { \ * it's difficult to tell apart from case #1. It should be used only in * limited contexts where the code knows that case #1 will never apply.) * + * * Separately allocated minimal tuple: t_data points MINIMAL_TUPLE_OFFSET + * bytes before the start of a MinimalTuple. As with the previous case, + * this can't be told apart from case #1 by inspection; code setting up + * or destroying this representation has to know what it's doing. + * * t_len should always be valid, except in the pointer-to-nothing case. * t_self and t_tableOid should be valid if the HeapTupleData points to * a disk buffer, or if it represents a copy of a tuple on disk. They |