diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 1e1e069a411e227de4b75b5be1a86ebc78c4b166..a3fecc4934c6f4f0e399062ac30db77f4c27641a 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.176 2008/05/01 19:55:40 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.177 2008/05/02 21:26:09 tgl Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
@@ -2238,6 +2238,26 @@ SELECT * FROM parent WHERE key = 2400;
</listitem>
</varlistentry>
+ <varlistentry id="guc-cursor-tuple-fraction" xreflabel="cursor_tuple_fraction">
+ <term><varname>cursor_tuple_fraction</varname> (<type>floating point</type>)</term>
+ <indexterm>
+ <primary><varname>cursor_tuple_fraction</> configuration parameter</primary>
+ </indexterm>
+ <listitem>
+ <para>
+ Sets the planner's estimate of the fraction of a cursor's rows that
+ will be retrieved. The default is 0.1. Smaller values of this
+ setting bias the planner towards using <quote>fast start</> plans
+ for cursors, which will retrieve the first few rows quickly while
+ perhaps taking a long time to fetch all rows. Larger values
+ put more emphasis on the total estimated time. At the maximum
+ setting of 1.0, cursors are planned exactly like regular queries,
+ considering only the total estimated time and not how soon the
+ first rows might be delivered.
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
<term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
<indexterm>
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 74e7258bba230e920e7417a85aa401aa0c1de47a..a6ad9dbdcec3c8b7d40eb8f1f8cbfb156830aa55 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.232 2008/04/17 21:22:14 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.233 2008/05/02 21:26:09 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -42,6 +42,9 @@
#include "utils/syscache.h"
+/* GUC parameter */
+double cursor_tuple_fraction = DEFAULT_CURSOR_TUPLE_FRACTION;
+
/* Hook for plugins to get control in planner() */
planner_hook_type planner_hook = NULL;
@@ -142,11 +145,23 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
{
/*
* We have no real idea how many tuples the user will ultimately FETCH
- * from a cursor, but it seems a good bet that he doesn't want 'em
- * all. Optimize for 10% retrieval (you gotta better number? Should
- * this be a SETtable parameter?)
+ * from a cursor, but it is often the case that he doesn't want 'em
+ * all, or would prefer a fast-start plan anyway so that he can
+ * process some of the tuples sooner. Use a GUC parameter to decide
+ * what fraction to optimize for.
+ */
+ tuple_fraction = cursor_tuple_fraction;
+
+ /*
+ * We document cursor_tuple_fraction as simply being a fraction,
+ * which means the edge cases 0 and 1 have to be treated specially
+ * here. We convert 1 to 0 ("all the tuples") and 0 to a very small
+ * fraction.
*/
- tuple_fraction = 0.10;
+ if (tuple_fraction >= 1.0)
+ tuple_fraction = 0.0;
+ else if (tuple_fraction <= 0.0)
+ tuple_fraction = 1e-10;
}
else
{
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 58b1a469acfd2703ad159ef5cbcb61508cb5ad81..993cf971a93088e7e070953ecc200f7a69f38b1f 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -10,7 +10,7 @@
* Written by Peter Eisentraut <peter_e@gmx.net>.
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.450 2008/05/01 19:55:40 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.451 2008/05/02 21:26:10 tgl Exp $
*
*--------------------------------------------------------------------
*/
@@ -1891,6 +1891,16 @@ static struct config_real ConfigureNamesReal[] =
DEFAULT_CPU_OPERATOR_COST, 0, DBL_MAX, NULL, NULL
},
+ {
+ {"cursor_tuple_fraction", PGC_USERSET, QUERY_TUNING_OTHER,
+ gettext_noop("Sets the planner's estimate of the fraction of "
+ "a cursor's rows that will be retrieved."),
+ NULL
+ },
+ &cursor_tuple_fraction,
+ DEFAULT_CURSOR_TUPLE_FRACTION, 0.0, 1.0, NULL, NULL
+ },
+
{
{"geqo_selection_bias", PGC_USERSET, QUERY_TUNING_GEQO,
gettext_noop("GEQO: selective pressure within the population."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 9fb567bd62b08d1efa544c005e819001f04d1d84..ce7845308320fe648ae1340dcaff66ef32f5045b 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -219,6 +219,7 @@
#default_statistics_target = 10 # range 1-1000
#constraint_exclusion = off
+#cursor_tuple_fraction = 0.1 # range 0.0-1.0
#from_collapse_limit = 8
#join_collapse_limit = 8 # 1 disables collapsing of explicit
# JOIN clauses
diff --git a/src/include/optimizer/planmain.h b/src/include/optimizer/planmain.h
index 74ba5131cc65d35e8843819a1e0afc6eeb61232c..0b46a173706bfacae48a97bf683b38882c0451c8 100644
--- a/src/include/optimizer/planmain.h
+++ b/src/include/optimizer/planmain.h
@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/include/optimizer/planmain.h,v 1.107 2008/04/17 21:22:14 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/planmain.h,v 1.108 2008/05/02 21:26:10 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -17,6 +17,10 @@
#include "nodes/plannodes.h"
#include "nodes/relation.h"
+/* GUC parameters */
+#define DEFAULT_CURSOR_TUPLE_FRACTION 0.1
+extern double cursor_tuple_fraction;
+
/*
* prototypes for plan/planmain.c
*/