[Doc] Improvements of tempbuffers doc

vbwagner · vbwagner · commit d87f8c8e6525 · 2016-12-12T17:31:39.000+03:00
Clareifed calculation of temp relation size
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -18865,6 +18865,16 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
         Shorthand for <literal>pg_relation_size(..., 'main')</literal>
        </entry>
       </row>
+      <row>
+       <entry>
+        <literal><function>pg_temp_relation_size(<parameter>relation</parameter> <type>regclass</type>)</function></literal>
+        </entry>
+       <entry><type>bigint</type></entry>
+       <entry>
+        Total space used by <literal>'main'</literal> fork of
+        the specified temporary table.
+       </entry>
+      </row>
       <row>
        <entry>
         <literal><function>pg_size_bytes(<type>text</type>)</function></literal>
@@ -19004,6 +19014,15 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     </itemizedlist>
    </para>
 
+   <para>
+    <function>pg_temp_relation_size</> accepts the OID or name of a temporary table
+    and returns the total size in bytes of <literal>'main'</literal> fork of
+    that relation.  We extend physical file for a temporary table when table doesn't
+    fit into <literal>temp_buffers</literal> cache anymore. So total and on-disk size of a temporary
+    table can differ. For other kinds of relations it will not throw any error,
+    just return NULL.
+   </para>
+
    <para>
     <function>pg_size_pretty</> can be used to format the result of one of
     the other functions in a human-readable way, using bytes, kB, MB, GB or TB
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
@@ -151,6 +151,15 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED | CONSTANT ] TABLE [
       table are automatically temporary as well.
      </para>
 
+     <para>
+     Unlike for permanent tables, we extend physical file for a temporary table
+     only when it doesn't fit into <literal>temp_buffers</> cache anymore.
+     So total and on-disk size of a temporary table can differ. Blocks are
+     not allocated in advance, so we may run out of free space on disk
+     when trying to evict a temp buffer from cache. It will cause an error.
+     This case is quite unusual, though.
+     </para>
+
      <para>
       The <link linkend="autovacuum">autovacuum daemon</link> cannot
       access and therefore cannot vacuum or analyze temporary tables.
diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
@@ -348,13 +348,19 @@ pg_temp_relation_size(PG_FUNCTION_ARGS)
 		PG_RETURN_NULL();
 
 	size = 0;
-	if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
+	if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP
+		&& rel->rd_rel->relkind == RELKIND_RELATION)
 	{
 		RelationOpenSmgr(rel);
 
 		if (rel->rd_smgr->smgr_main_nblocks != InvalidBlockNumber)
 			size = BLCKSZ * rel->rd_smgr->smgr_main_nblocks;
 	}
+	else
+	{
+		relation_close(rel, AccessShareLock);
+		PG_RETURN_NULL();
+	}
 
 	relation_close(rel, AccessShareLock);
 
