Update index cost estimation docs to final 7.0 scheme.

Author: tglsfdc

doc/src/sgml/indexcost.sgml

@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/Attic/indexcost.sgml,v 2.2 2000/03/31 03:27:40 thomas Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/Attic/indexcost.sgml,v 2.3 2000/03/31 17:18:26 tgl Exp $
 -->
 
  <chapter>
@@ -14,20 +14,12 @@ $Header: /cvsroot/pgsql/doc/src/sgml/Attic/indexcost.sgml,v 2.2 2000/03/31 03:27
    </para>
   </note>
 
-<!--
-I have written the attached bit of doco about the new index cost
-estimator procedure definition, but I am not sure where to put it.
-There isn't (AFAICT) any existing documentation about how to make
-a new kind of index, which would be the proper place for it.
-May I impose on you to find/make a place for this and mark it up
-properly?
-
-Also, doc/src/graphics/catalogs.ag needs to be updated, but I have
-no idea how.  (The amopselect and amopnpages fields of pg_amop
-are gone; pg_am has a new field amcostestimate.)
-
-			regards, tom lane
--->
+   <note>
+    <para>
+     This must eventually become part of a much larger chapter about
+     writing new index access methods.
+    </para>
+   </note>
 
   <para>
    Every index access method must provide a cost estimation function for
@@ -64,7 +56,8 @@ amcostestimate (Query *root,
                 RelOptInfo *rel,
                 IndexOptInfo *index,
                 List *indexQuals,
-                Cost *indexAccessCost,
+                Cost *indexStartupCost,
+                Cost *indexTotalCost,
                 Selectivity *indexSelectivity);
    </programlisting>
 
@@ -111,14 +104,23 @@ amcostestimate (Query *root,
   </para>
 
   <para>
-   The last two parameters are pass-by-reference outputs:
+   The last three parameters are pass-by-reference outputs:
 
    <variablelist>
     <varlistentry>
-     <term>*indexAccessCost</term>
+     <term>*indexStartupCost</term>
      <listitem>
       <para>
-       Set to cost of index processing.
+       Set to cost of index startup processing
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term>*indexTotalCost</term>
+     <listitem>
+      <para>
+       Set to total cost of index processing
       </para>
      </listitem>
     </varlistentry>
@@ -141,15 +143,29 @@ amcostestimate (Query *root,
   </para>
 
   <para>
-   The indexAccessCost should be computed in the units used by
-   src/backend/optimizer/path/costsize.c: a disk block fetch has cost 1.0,
-   and the cost of processing one index tuple should usually be taken as
-   cpu_index_page_weight (which is a user-adjustable optimizer parameter).
-   The access cost should include all disk and CPU costs associated with
-   scanning the index itself, but NOT the cost of retrieving or processing
+   The index access costs should be computed in the units used by
+   src/backend/optimizer/path/costsize.c: a sequential disk block fetch
+   has cost 1.0, a nonsequential fetch has cost random_page_cost, and
+   the cost of processing one index tuple should usually be taken as
+   cpu_index_tuple_cost (which is a user-adjustable optimizer parameter).
+   In addition, an appropriate multiple of cpu_operator_cost should be charged
+   for any comparison operators invoked during index processing (especially
+   evaluation of the indexQuals themselves).
+  </para>
+
+  <para>
+   The access costs should include all disk and CPU costs associated with
+   scanning the index itself, but NOT the costs of retrieving or processing
    the main-table tuples that are identified by the index.
   </para>
 
+  <para>
+   The "startup cost" is the part of the total scan cost that must be expended
+   before we can begin to fetch the first tuple.  For most indexes this can
+   be taken as zero, but an index type with a high startup cost might want
+   to set it nonzero.
+  </para>
+
   <para>
    The indexSelectivity should be set to the estimated fraction of the main
    table tuples that will be retrieved during the index scan.  In the case
@@ -167,10 +183,11 @@ amcostestimate (Query *root,
     <para>
      Estimate and return the fraction of main-table tuples that will be visited
      based on the given qual conditions.  In the absence of any index-type-specific
-     knowledge, use the standard optimizer function clauselist_selec():
+     knowledge, use the standard optimizer function clauselist_selectivity():
 
      <programlisting>
-*indexSelectivity = clauselist_selec(root, indexQuals);
+*indexSelectivity = clauselist_selectivity(root, indexQuals,
+                                           lfirsti(rel->relids));
      </programlisting>
     </para>
    </step>
@@ -193,10 +210,18 @@ amcostestimate (Query *root,
 
    <step>
     <para>
-     Compute the index access cost as
+     Compute the index access cost.  A generic estimator might do this:
 
      <programlisting>
-*indexAccessCost = numIndexPages + cpu_index_page_weight * numIndexTuples;
+    /*
+     * Our generic assumption is that the index pages will be read
+     * sequentially, so they have cost 1.0 each, not random_page_cost.
+     * Also, we charge for evaluation of the indexquals at each index tuple.
+     * All the costs are assumed to be paid incrementally during the scan.
+     */
+    *indexStartupCost = 0;
+    *indexTotalCost = numIndexPages +
+        (cpu_index_tuple_cost + cost_qual_eval(indexQuals)) * numIndexTuples;
      </programlisting>
     </para>
    </step>
@@ -213,8 +238,8 @@ amcostestimate (Query *root,
 
    <programlisting>
 prorettype = 0
-pronargs = 6
-proargtypes = 0 0 0 0 0 0
+pronargs = 7
+proargtypes = 0 0 0 0 0 0 0
    </programlisting>
 
    We use zero ("opaque") for all the arguments since none of them have types