Update comments for PG_DETOAST_PACKED and VARDATA_ANY on a structures

that require alignment.

Add a paragraph to the "User-Defined Types" chapter on using these
macros since it seems like they're a hit.

Gregory Stark

1 files changed, 31 insertions, 13 deletions

diff --git a/doc/src/sgml/xtypes.sgml b/doc/src/sgml/xtypes.sgml
index e1ec4ea189b..bc84ccd1c4b 100644
--- a/doc/src/sgml/xtypes.sgml
+++ b/doc/src/sgml/xtypes.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xtypes.sgml,v 1.28 2006/09/16 00:30:16 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xtypes.sgml,v 1.29 2007/05/15 17:39:54 momjian Exp $ -->
 
  <sect1 id="xtypes">
   <title>User-Defined Types</title>
@@ -237,20 +237,38 @@ CREATE TYPE complex (
     <primary>TOAST</primary>
     <secondary>and user-defined types</secondary>
    </indexterm>
-  If the values of your data type might exceed a few hundred bytes in
-  size (in internal form), you should make the data type
-  <acronym>TOAST</>-able (see <xref linkend="storage-toast">).
-  To do this, the internal
-  representation must follow the standard layout for variable-length
-  data: the first four bytes must be an <type>int32</type> containing
-  the total length in bytes of the datum (including itself).  The C
-  functions operating on the data type must be careful to unpack any
+  If the values of your data type vary in size (in internal form), you should
+  make the data type <acronym>TOAST</>-able (see <xref
+  linkend="storage-toast">). You should do this even if the data are always
+  too small to be compressed or stored externally because
+  <productname>Postgres</> can save space on small data using
+  <acronym>TOAST</> as well. 
+ </para>
+
+ <para>
+  To do this, the internal representation must follow the standard layout for
+  variable-length data: the first four bytes must be an <type>int32</type>
+  which is never accessed directly (customarily named <literal>vl_len_</>). You
+  must use <function>SET_VARSIZE()</function> to store the size of the datum
+  in this field and <function>VARSIZE()</function> to retrieve it. The C
+  functions operating on the data type must always be careful to unpack any
   toasted values they are handed, by using <function>PG_DETOAST_DATUM</>.
   (This detail is customarily hidden by defining type-specific
-  <function>GETARG</function> macros.) Then, 
-  when running the <command>CREATE TYPE</command> command, specify the
-  internal length as <literal>variable</> and select the appropriate
-  storage option.
+  <function>GETARG_DATATYPE_P</function> macros.) Then, when running the
+  <command>CREATE TYPE</command> command, specify the internal length as
+  <literal>variable</> and select the appropriate storage option.
+ </para>
+
+ <para>
+  If the alignment is unimportant (either just for a specific function or
+  because the data type specifies byte alignment anyways) then it's possible
+  to avoid some of the overhead of <function>PG_DETOAST_DATUM</>. You can use
+  <function>PG_DETOAST_DATUM_PACKED</> instead (customarily hidden by
+  defining a <function>GETARG_DATATYPE_PP</> macro) and using the macros
+  <function>VARSIZE_ANY_EXHDR</> and <function>VARDATA_ANY</> macros.
+  Again, the data returned by these macros is not aligned even if the data
+  type definition specifies an alignment. If the alignment is important you
+  must go through the regular <function>PG_DETOAST_DATUM</> interface.
  </para>
 
  <para>