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 002402c1ad623eefff983284cb1add72d6c5f6fc..bda49f6c42af3db4736239cb9143857d2f7a4e5b 100644 (file)
--- a/doc/src/sgml/storage.sgml
+++ b/doc/src/sgml/storage.sgml
@@ -452,9 +452,7 @@ 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 09b9edda45609f4e74a08d3159a4ad24dea8e1dc..2a71988bdc0109b92e82f17045a2ea9d980e1533 100644 (file)
--- a/src/backend/access/common/heaptuple.c
+++ b/src/backend/access/common/heaptuple.c
@@ -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 */

diff --git a/src/backend/utils/adt/rangetypes.c b/src/backend/utils/adt/rangetypes.c
index 01ad8bc240b43b025555996e6c82e06c2553ded2..f7a306669ab497923cd9410a7be0bbc204bbd789 100644 (file)
--- a/src/backend/utils/adt/rangetypes.c
+++ b/src/backend/utils/adt/rangetypes.c
@@ -2384,7 +2384,8 @@ 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? */