Sophie: postgresql8.4-docs-8.4.12-0.1mdv2010.2 i586

postgresql8.4-docs-8.4.12-0.1mdv2010.2.i586.rpm

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>PostgreSQL 8.4.12 Documentation</TH
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>Chapter 52. GIN Indexes</TD
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><H1
CLASS="SECT1"
><A
NAME="GIN-EXTENSIBILITY"
>52.2. Extensibility</A
></H1
><P
>   The <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> interface has a high level of abstraction,
   requiring the access method implementer only to implement the semantics of
   the data type being accessed.  The <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> layer itself
   takes care of concurrency, logging and searching the tree structure.
 </P
><P
>   All it takes to get a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> access method working is to
   implement four (or five) user-defined methods, which define the behavior of
   keys in the tree and the relationships between keys, indexed values,
   and indexable queries. In short, <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> combines
   extensibility with generality, code reuse, and a clean interface.
 </P
><P
>   The four methods that an operator class for
   <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> must provide are:
 </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>int compare(Datum a, Datum b)</DT
><DD
><P
>       Compares keys (not indexed values!) and returns an integer less than
       zero, zero, or greater than zero, indicating whether the first key is
       less than, equal to, or greater than the second.
      </P
></DD
><DT
>Datum *extractValue(Datum inputValue, int32 *nkeys)</DT
><DD
><P
>       Returns an array of keys given a value to be indexed.  The
       number of returned keys must be stored into <TT
CLASS="LITERAL"
>*nkeys</TT
>.
      </P
></DD
><DT
>Datum *extractQuery(Datum query, int32 *nkeys,
        StrategyNumber n, bool **pmatch, Pointer **extra_data)</DT
><DD
><P
>       Returns an array of keys given a value to be queried; that is,
       <TT
CLASS="LITERAL"
>query</TT
> is the value on the right-hand side of an
       indexable operator whose left-hand side is the indexed column.
       <TT
CLASS="LITERAL"
>n</TT
> is the strategy number of the operator within the
       operator class (see <A
HREF="xindex.html#XINDEX-STRATEGIES"
>Section 34.14.2</A
>).
       Often, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> will need
       to consult <TT
CLASS="LITERAL"
>n</TT
> to determine the data type of
       <TT
CLASS="LITERAL"
>query</TT
> and the key values that need to be extracted.
       The number of returned keys must be stored into <TT
CLASS="LITERAL"
>*nkeys</TT
>.
       If the query contains no keys then <CODE
CLASS="FUNCTION"
>extractQuery</CODE
>
       should store 0 or -1 into <TT
CLASS="LITERAL"
>*nkeys</TT
>, depending on the
       semantics of the operator.  0 means that every
       value matches the <TT
CLASS="LITERAL"
>query</TT
> and a full-index scan should be
       performed (but see <A
HREF="gin-limit.html"
>Section 52.5</A
>).
       -1 means that nothing can match the <TT
CLASS="LITERAL"
>query</TT
>, and
       so the index scan can be skipped entirely.
       <TT
CLASS="LITERAL"
>pmatch</TT
> is an output argument for use when partial match
       is supported.  To use it, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> must allocate
       an array of <TT
CLASS="LITERAL"
>*nkeys</TT
> booleans and store its address at
       <TT
CLASS="LITERAL"
>*pmatch</TT
>.  Each element of the array should be set to TRUE
       if the corresponding key requires partial match, FALSE if not.
       If <TT
CLASS="LITERAL"
>*pmatch</TT
> is set to NULL then GIN assumes partial match
       is not required.  The variable is initialized to NULL before call,
       so this argument can simply be ignored by operator classes that do
       not support partial match.
       <TT
CLASS="LITERAL"
>extra_data</TT
> is an output argument that allows
       <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> to pass additional data to the
       <CODE
CLASS="FUNCTION"
>consistent</CODE
> and <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> methods.
       To use it, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> must allocate
       an array of <TT
CLASS="LITERAL"
>*nkeys</TT
> Pointers and store its address at
       <TT
CLASS="LITERAL"
>*extra_data</TT
>, then store whatever it wants to into the
       individual pointers.  The variable is initialized to NULL before
       call, so this argument can simply be ignored by operator classes that
       do not require extra data.  If <TT
CLASS="LITERAL"
>*extra_data</TT
> is set, the
       whole array is passed to the <CODE
CLASS="FUNCTION"
>consistent</CODE
> method, and
       the appropriate element to the <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> method.
      </P
></DD
><DT
>bool consistent(bool check[], StrategyNumber n, Datum query,
                           int32 nkeys, Pointer extra_data[], bool *recheck)</DT
><DD
><P
>       Returns TRUE if the indexed value satisfies the query operator with
       strategy number <TT
CLASS="LITERAL"
>n</TT
> (or might satisfy, if the recheck
       indication is returned).  The <TT
CLASS="LITERAL"
>check</TT
> array has length
       <TT
CLASS="LITERAL"
>nkeys</TT
>, which is the same as the number of keys previously
       returned by <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> for this <TT
CLASS="LITERAL"
>query</TT
> datum.
       Each element of the
       <TT
CLASS="LITERAL"
>check</TT
> array is TRUE if the indexed value contains the
       corresponding query key, ie, if (check[i] == TRUE) the i-th key of the
       <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> result array is present in the indexed value.
       The original <TT
CLASS="LITERAL"
>query</TT
> datum (not the extracted key array!) is
       passed in case the <CODE
CLASS="FUNCTION"
>consistent</CODE
> method needs to consult it.
       <TT
CLASS="LITERAL"
>extra_data</TT
> is the extra-data array returned by
       <CODE
CLASS="FUNCTION"
>extractQuery</CODE
>, or NULL if none.
       On success, <TT
CLASS="LITERAL"
>*recheck</TT
> should be set to TRUE if the heap
       tuple needs to be rechecked against the query operator, or FALSE if
       the index test is exact.
      </P
></DD
></DL
></DIV
><P
>  Optionally, an operator class for
  <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> can supply a fifth method:
 </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>int comparePartial(Datum partial_key, Datum key, StrategyNumber n,
                              Pointer extra_data)</DT
><DD
><P
>       Compare a partial-match query to an index key.  Returns an integer
       whose sign indicates the result: less than zero means the index key
       does not match the query, but the index scan should continue; zero
       means that the index key does match the query; greater than zero
       indicates that the index scan should stop because no more matches
       are possible.  The strategy number <TT
CLASS="LITERAL"
>n</TT
> of the operator
       that generated the partial match query is provided, in case its
       semantics are needed to determine when to end the scan.  Also,
       <TT
CLASS="LITERAL"
>extra_data</TT
> is the corresponding element of the extra-data
       array made by <CODE
CLASS="FUNCTION"
>extractQuery</CODE
>, or NULL if none.
      </P
></DD
></DL
></DIV
><P
>  To support <SPAN
CLASS="QUOTE"
>"partial match"</SPAN
> queries, an operator class must
  provide the <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> method, and its
  <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> method must set the <TT
CLASS="LITERAL"
>pmatch</TT
>
  parameter when a partial-match query is encountered.  See
  <A
HREF="gin-implementation.html#GIN-PARTIAL-MATCH"
>Section 52.3.2</A
> for details.
 </P
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