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>Chapter 56. Writing A Table Sampling Method</H1
><P
>  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s implementation of the <TT
CLASS="LITERAL"
>TABLESAMPLE</TT
>
  clause supports custom table sampling methods, in addition to
  the <TT
CLASS="LITERAL"
>BERNOULLI</TT
> and <TT
CLASS="LITERAL"
>SYSTEM</TT
> methods that are required
  by the SQL standard.  The sampling method determines which rows of the
  table will be selected when the <TT
CLASS="LITERAL"
>TABLESAMPLE</TT
> clause is used.
 </P
><P
>  At the SQL level, a table sampling method is represented by a single SQL
  function, typically implemented in C, having the signature
</P><PRE
CLASS="PROGRAMLISTING"
>method_name(internal) RETURNS tsm_handler</PRE
><P>
  The name of the function is the same method name appearing in the
  <TT
CLASS="LITERAL"
>TABLESAMPLE</TT
> clause.  The <TT
CLASS="TYPE"
>internal</TT
> argument is a dummy
  (always having value zero) that simply serves to prevent this function from
  being called directly from a SQL command.
  The result of the function must be a palloc'd struct of
  type <TT
CLASS="TYPE"
>TsmRoutine</TT
>, which contains pointers to support functions for
  the sampling method.  These support functions are plain C functions and
  are not visible or callable at the SQL level.  The support functions are
  described in <A
HREF="tablesample-support-functions.html"
>Section 56.1</A
>.
 </P
><P
>  In addition to function pointers, the <TT
CLASS="TYPE"
>TsmRoutine</TT
> struct must
  provide these additional fields:
 </P
><P
></P
><DIV
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><DL
><DT
><TT
CLASS="LITERAL"
>List *parameterTypes</TT
></DT
><DD
><P
>     This is an OID list containing the data type OIDs of the parameter(s)
     that will be accepted by the <TT
CLASS="LITERAL"
>TABLESAMPLE</TT
> clause when this
     sampling method is used.  For example, for the built-in methods, this
     list contains a single item with value <TT
CLASS="LITERAL"
>FLOAT4OID</TT
>, which
     represents the sampling percentage.  Custom sampling methods can have
     more or different parameters.
    </P
></DD
><DT
><TT
CLASS="LITERAL"
>bool repeatable_across_queries</TT
></DT
><DD
><P
>     If <TT
CLASS="LITERAL"
>true</TT
>, the sampling method can deliver identical samples
     across successive queries, if the same parameters
     and <TT
CLASS="LITERAL"
>REPEATABLE</TT
> seed value are supplied each time and the
     table contents have not changed.  When this is <TT
CLASS="LITERAL"
>false</TT
>,
     the <TT
CLASS="LITERAL"
>REPEATABLE</TT
> clause is not accepted for use with the
     sampling method.
    </P
></DD
><DT
><TT
CLASS="LITERAL"
>bool repeatable_across_scans</TT
></DT
><DD
><P
>     If <TT
CLASS="LITERAL"
>true</TT
>, the sampling method can deliver identical samples
     across successive scans in the same query (assuming unchanging
     parameters, seed value, and snapshot).
     When this is <TT
CLASS="LITERAL"
>false</TT
>, the planner will not select plans that
     would require scanning the sampled table more than once, since that
     might result in inconsistent query output.
    </P
></DD
></DL
></DIV
><P
>  The <TT
CLASS="TYPE"
>TsmRoutine</TT
> struct type is declared
  in <TT
CLASS="FILENAME"
>src/include/access/tsmapi.h</TT
>, which see for additional
  details.
 </P
><P
>  The table sampling methods included in the standard distribution are good
  references when trying to write your own.  Look into
  the <TT
CLASS="FILENAME"
>src/backend/access/tablesample</TT
> subdirectory of the source
  tree for the built-in sampling methods, and into the <TT
CLASS="FILENAME"
>contrib</TT
>
  subdirectory for add-on methods.
 </P
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