From: Bruce Momjian <bruce@momjian.us>
Date: Tue, 31 Oct 2023 13:10:34 +0000 (-0400)
Subject: doc:  1-byte varlena headers can be used for user PLAIN storage
X-Git-Tag: REL_11_22~15
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=55ce8fc3998a899d09038fb2bb31874e9894a7b1;p=thirdparty%2Fpostgresql.git

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

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
---

diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
index 39d90ce753d..c8f9fc3d03f 100644
--- a/doc/src/sgml/storage.sgml
+++ b/doc/src/sgml/storage.sgml
@@ -444,9 +444,7 @@ 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>
diff --git a/src/backend/access/common/heaptuple.c b/src/backend/access/common/heaptuple.c
index 340fe81a1eb..30459c19a7b 100644
--- a/src/backend/access/common/heaptuple.c
+++ b/src/backend/access/common/heaptuple.c
@@ -69,7 +69,16 @@
 #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 != 'p')
 /* Use this if it's already known varlena */
diff --git a/src/backend/utils/adt/rangetypes.c b/src/backend/utils/adt/rangetypes.c
index 558e7becc4e..71d2ef0a0a7 100644
--- a/src/backend/utils/adt/rangetypes.c
+++ b/src/backend/utils/adt/rangetypes.c
@@ -2383,7 +2383,8 @@ range_contains_elem_internal(TypeCacheEntry *typcache, RangeType *r, Datum val)
  * 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? */