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doc: 1-byte varlena headers can be used for user PLAIN storage

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This also updates some C comments.

Reported-by: suchithjn22@gmail.com

Discussion: https://postgr.es/m/167336599095.2667301.15497893107226841625@wrigleys.postgresql.org

Author: Laurenz Albe (doc patch)

Backpatch-through: 11
This commit is contained in:
Bruce Momjian 2023-10-31 09:10:35 -04:00
parent 75e700db45
commit 989adace3f
3 changed files with 13 additions and 5 deletions
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4
doc/src/sgml/storage.sgml
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@ -456,9 +456,7 @@ for storing <acronym>TOAST</acronym>-able columns on disk:
<listitem>
<para>
<literal>PLAIN</literal> prevents either compression or
out-of-line storage; furthermore it disables use of single-byte headers
for varlena types.
This is the only possible strategy for
out-of-line storage. This is the only possible strategy for
columns of non-<acronym>TOAST</acronym>-able data types.
</para>
</listitem>

11
src/backend/access/common/heaptuple.c
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@ -68,7 +68,16 @@
#include "utils/memutils.h"
/* Does att's datatype allow packing into the 1-byte-header varlena format? */
/*
* Does att's datatype allow packing into the 1-byte-header varlena format?
* While functions that use TupleDescAttr() and assign attstorage =
* TYPSTORAGE_PLAIN cannot use packed varlena headers, functions that call
* TupleDescInitEntry() use typeForm->typstorage (TYPSTORAGE_EXTENDED) and
* can use packed varlena headers, e.g.:
* CREATE TABLE test(a VARCHAR(10000) STORAGE PLAIN);
* INSERT INTO test VALUES (repeat('A',10));
* This can be verified with pageinspect.
*/
#define ATT_IS_PACKABLE(att) \
((att)->attlen == -1 && (att)->attstorage != TYPSTORAGE_PLAIN)
/* Use this if it's already known varlena */

3
src/backend/utils/adt/rangetypes.c
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@ -2608,7 +2608,8 @@ range_contains_elem_internal(TypeCacheEntry *typcache, const RangeType *r, Datum
* values into a range object. They are modeled after heaptuple.c's
* heap_compute_data_size() and heap_fill_tuple(), but we need not handle
* null values here. TYPE_IS_PACKABLE must test the same conditions as
* heaptuple.c's ATT_IS_PACKABLE macro.
* heaptuple.c's ATT_IS_PACKABLE macro. See the comments thare for more
* details.
*/
/* Does datatype allow packing into the 1-byte-header varlena format? */