Update comments for PG_DETOAST_PACKED and VARDATA_ANY on a structures

bmomjian · bmomjian · commit 178214d2ae46 · 2007-05-15T17:39:54.000Z
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
diff --git 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>
diff --git a/src/include/fmgr.h b/src/include/fmgr.h
@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/fmgr.h,v 1.50 2007/04/06 04:21:44 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/fmgr.h,v 1.51 2007/05/15 17:39:54 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -158,6 +158,12 @@ extern void fmgr_info_copy(FmgrInfo *dstinfo, FmgrInfo *srcinfo,
  * The resulting datum can be accessed using VARSIZE_ANY() and VARDATA_ANY()
  * (beware of multiple evaluations in those macros!)
  *
+ * WARNING: It is only safe to use PG_DETOAST_DATUM_UNPACKED() and
+ * VARDATA_ANY() if you really don't care about the alignment. Either because
+ * you're working with something like text where the alignment doesn't matter
+ * or because you're not going to access its constituent parts and just use
+ * things like memcpy on it anyways.
+ *
  * Note: it'd be nice if these could be macros, but I see no way to do that
  * without evaluating the arguments multiple times, which is NOT acceptable.
  */
@@ -174,6 +180,7 @@ extern struct varlena *pg_detoast_datum_packed(struct varlena * datum);
 #define PG_DETOAST_DATUM_SLICE(datum,f,c) \
 		pg_detoast_datum_slice((struct varlena *) DatumGetPointer(datum), \
 		(int32) f, (int32) c)
+/* WARNING -- unaligned pointer */
 #define PG_DETOAST_DATUM_PACKED(datum) \
 	pg_detoast_datum_packed((struct varlena *) DatumGetPointer(datum))
 
diff --git a/src/include/postgres.h b/src/include/postgres.h
@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1995, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/postgres.h,v 1.80 2007/05/04 02:01:02 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/postgres.h,v 1.81 2007/05/15 17:39:54 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -235,6 +235,12 @@ typedef struct
  * use VARSIZE_ANY/VARSIZE_ANY_EXHDR/VARDATA_ANY.  The other macros here
  * should usually be used only by tuple assembly/disassembly code and
  * code that specifically wants to work with still-toasted Datums.
+ *
+ * WARNING: It is only safe to use VARDATA_ANY() -- typically with
+ * PG_DETOAST_DATUM_UNPACKED() -- if you really don't care about the alignment.
+ * Either because you're working with something like text where the alignment
+ * doesn't matter or because you're not going to access its constituent parts
+ * and just use things like memcpy on it anyways.
  */
 #define VARDATA(PTR)						VARDATA_4B(PTR)
 #define VARSIZE(PTR)						VARSIZE_4B(PTR)
@@ -265,6 +271,7 @@ typedef struct
 	  VARSIZE_4B(PTR)-4))
 
 /* caution: this will not work on an external or compressed-in-line Datum */
+/* caution: this will return a possibly unaligned pointer */
 #define VARDATA_ANY(PTR) \
 	 (VARATT_IS_1B(PTR) ? VARDATA_1B(PTR) : VARDATA_4B(PTR))
 
