Sophie: postgresql8.3-docs-8.3.6-2mdv2008.1 x86

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>PostgreSQL 8.3.6 Documentation</TH
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>Chapter 52. GIN Indexes</TD
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><H1
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><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 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 index operator class for
   <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> must provide are:
 </P
><P
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><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)</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 number of keys is equal to zero then <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> 
       should store 0 or -1 into <TT
CLASS="LITERAL"
>*nkeys</TT
>. 0 means that any 
       row matches the <TT
CLASS="LITERAL"
>query</TT
> and sequence scan should be 
       produced. -1 means nothing can satisfy <TT
CLASS="LITERAL"
>query</TT
>. 
       Choice of value should be based on semantics meaning of operation with 
       given strategy number.
      </P
></DD
><DT
>bool consistent(bool check[], StrategyNumber n, Datum query)</DT
><DD
><P
>       Returns TRUE if the indexed value satisfies the query operator with
       strategy number <TT
CLASS="LITERAL"
>n</TT
> (or would satisfy, if the operator is
       marked RECHECK in the operator class).  The <TT
CLASS="LITERAL"
>check</TT
> array has
       the same length as the number of keys previously returned by
       <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> for this query.  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.
      </P
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