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>Chapter 49. GiST Indexes</TD
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>49.3. Implementation</A
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> There are seven methods that an index operator class for
<ACRONYM
CLASS="ACRONYM"
>GiST</ACRONYM
> must provide:
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>consistent</DT
><DD
><P
> Given a predicate <TT
CLASS="LITERAL"
>p</TT
> on a tree page, and a user
query, <TT
CLASS="LITERAL"
>q</TT
>, this method will return false if it is
certain that both <TT
CLASS="LITERAL"
>p</TT
> and <TT
CLASS="LITERAL"
>q</TT
> cannot
be true for a given data item.
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>union</DT
><DD
><P
> This method consolidates information in the tree. Given a set of
entries, this function generates a new predicate that is true for all
the entries.
</P
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><DT
>compress</DT
><DD
><P
> Converts the data item into a format suitable for physical storage in
an index page.
</P
></DD
><DT
>decompress</DT
><DD
><P
> The reverse of the <CODE
CLASS="FUNCTION"
>compress</CODE
> method. Converts the
index representation of the data item into a format that can be
manipulated by the database.
</P
></DD
><DT
>penalty</DT
><DD
><P
> Returns a value indicating the <SPAN
CLASS="QUOTE"
>"cost"</SPAN
> of inserting the new
entry into a particular branch of the tree. items will be inserted
down the path of least <CODE
CLASS="FUNCTION"
>penalty</CODE
> in the tree.
</P
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><DT
>picksplit</DT
><DD
><P
> When a page split is necessary, this function decides which entries on
the page are to stay on the old page, and which are to move to the new
page.
</P
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><DT
>same</DT
><DD
><P
> Returns true if two entries are identical, false otherwise.
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