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>Chapter 50. Index Access Method Interface Definition</TD
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><H1
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><A
NAME="INDEX-COST-ESTIMATION"
>50.6. Index Cost Estimation Functions</A
></H1
><P
>   The amcostestimate function is given a list of WHERE clauses that have
   been determined to be usable with the index.  It must return estimates
   of the cost of accessing the index and the selectivity of the WHERE
   clauses (that is, the fraction of parent-table rows that will be
   retrieved during the index scan).  For simple cases, nearly all the
   work of the cost estimator can be done by calling standard routines
   in the optimizer; the point of having an amcostestimate function is
   to allow index access methods to provide index-type-specific knowledge,
   in case it is possible to improve on the standard estimates.
  </P
><P
>   Each amcostestimate function must have the signature:

</P><PRE
CLASS="PROGRAMLISTING"
>void
amcostestimate (PlannerInfo *root,
                IndexOptInfo *index,
                List *indexQuals,
                RelOptInfo *outer_rel,
                Cost *indexStartupCost,
                Cost *indexTotalCost,
                Selectivity *indexSelectivity,
                double *indexCorrelation);</PRE
><P>

   The first four parameters are inputs:

   <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>root</DT
><DD
><P
>       The planner's information about the query being processed.
      </P
></DD
><DT
>index</DT
><DD
><P
>       The index being considered.
      </P
></DD
><DT
>indexQuals</DT
><DD
><P
>       List of index qual clauses (implicitly ANDed);
       a NIL list indicates no qualifiers are available.
       Note that the list contains expression trees, not ScanKeys.
      </P
></DD
><DT
>outer_rel</DT
><DD
><P
>       If the index is being considered for use in a join inner indexscan,
       the planner's information about the outer side of the join.  Otherwise
       NULL.  When non-NULL, some of the qual clauses will be join clauses
       with this rel rather than being simple restriction clauses.  Also,
       the cost estimator should expect that the index scan will be repeated
       for each row of the outer rel.
      </P
></DD
></DL
></DIV
><P>
  </P
><P
>   The last four parameters are pass-by-reference outputs:

   <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>*indexStartupCost</DT
><DD
><P
>       Set to cost of index start-up processing
      </P
></DD
><DT
>*indexTotalCost</DT
><DD
><P
>       Set to total cost of index processing
      </P
></DD
><DT
>*indexSelectivity</DT
><DD
><P
>       Set to index selectivity
      </P
></DD
><DT
>*indexCorrelation</DT
><DD
><P
>       Set to correlation coefficient between index scan order and
       underlying table's order
      </P
></DD
></DL
></DIV
><P>
  </P
><P
>   Note that cost estimate functions must be written in C, not in SQL or
   any available procedural language, because they must access internal
   data structures of the planner/optimizer.
  </P
><P
>   The index access costs should be computed using the parameters used by
   <TT
CLASS="FILENAME"
>src/backend/optimizer/path/costsize.c</TT
>: a sequential
   disk block fetch has cost <TT
CLASS="VARNAME"
>seq_page_cost</TT
>, a nonsequential fetch
   has cost <TT
CLASS="VARNAME"
>random_page_cost</TT
>, and the cost of processing one index
   row should usually be taken as <TT
CLASS="VARNAME"
>cpu_index_tuple_cost</TT
>.  In
   addition, an appropriate multiple of <TT
CLASS="VARNAME"
>cpu_operator_cost</TT
> should
   be charged for any comparison operators invoked during index processing
   (especially evaluation of the indexQuals themselves).
  </P
><P
>   The access costs should include all disk and CPU costs associated with
   scanning the index itself, but <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> the costs of retrieving or
   processing the parent-table rows that are identified by the index.
  </P
><P
>   The <SPAN
CLASS="QUOTE"
>"start-up cost"</SPAN
> is the part of the total scan cost that
   must be expended before we can begin to fetch the first row.  For most
   indexes this can be taken as zero, but an index type with a high start-up
   cost might want to set it nonzero.
  </P
><P
>   The indexSelectivity should be set to the estimated fraction of the parent
   table rows that will be retrieved during the index scan.  In the case
   of a lossy query, this will typically be higher than the fraction of
   rows that actually pass the given qual conditions.
  </P
><P
>   The indexCorrelation should be set to the correlation (ranging between
   -1.0 and 1.0) between the index order and the table order.  This is used
   to adjust the estimate for the cost of fetching rows from the parent
   table.
  </P
><P
>   In the join case, the returned numbers should be averages expected for
   any one scan of the index.
  </P
><DIV
CLASS="PROCEDURE"
><P
><B
>Cost Estimation</B
></P
><P
>    A typical cost estimator will proceed as follows:
   </P
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
>     Estimate and return the fraction of parent-table rows that will be visited
     based on the given qual conditions.  In the absence of any index-type-specific
     knowledge, use the standard optimizer function <CODE
CLASS="FUNCTION"
>clauselist_selectivity()</CODE
>:

</P><PRE
CLASS="PROGRAMLISTING"
>*indexSelectivity = clauselist_selectivity(root, indexQuals,
                                           index-&gt;rel-&gt;relid,
                                           JOIN_INNER, NULL);</PRE
><P>
    </P
></LI
><LI
CLASS="STEP"
><P
>     Estimate the number of index rows that will be visited during the
     scan.  For many index types this is the same as indexSelectivity times
     the number of rows in the index, but it might be more.  (Note that the
     index's size in pages and rows is available from the IndexOptInfo struct.)
    </P
></LI
><LI
CLASS="STEP"
><P
>     Estimate the number of index pages that will be retrieved during the scan.
     This might be just indexSelectivity times the index's size in pages.
    </P
></LI
><LI
CLASS="STEP"
><P
>     Compute the index access cost.  A generic estimator might do this:

</P><PRE
CLASS="PROGRAMLISTING"
>    /*
     * Our generic assumption is that the index pages will be read
     * sequentially, so they cost seq_page_cost each, not random_page_cost.
     * Also, we charge for evaluation of the indexquals at each index row.
     * All the costs are assumed to be paid incrementally during the scan.
     */
    cost_qual_eval(&amp;index_qual_cost, indexQuals, root);
    *indexStartupCost = index_qual_cost.startup;
    *indexTotalCost = seq_page_cost * numIndexPages +
        (cpu_index_tuple_cost + index_qual_cost.per_tuple) * numIndexTuples;</PRE
><P>

     However, the above does not account for amortization of index reads
     across repeated index scans in the join case.
    </P
></LI
><LI
CLASS="STEP"
><P
>     Estimate the index correlation.  For a simple ordered index on a single
     field, this can be retrieved from pg_statistic.  If the correlation
     is not known, the conservative estimate is zero (no correlation).
    </P
></LI
></OL
></DIV
><P
>   Examples of cost estimator functions can be found in
   <TT
CLASS="FILENAME"
>src/backend/utils/adt/selfuncs.c</TT
>.
  </P
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