PostgreSQL 8.3.21 Documentation
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# 51.3. Implementation

There are seven methods that an index operator class for GiST must provide:

consistent

  Given a predicate `p` on a tree page, and a user query, `q`, this method will return false if it is certain that both `p` and `q` cannot be true for a given data item.

union

  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.

compress

  Converts the data item into a format suitable for physical storage in an index page.

decompress

  The reverse of the `compress` method. Converts the index representation of the data item into a format that can be manipulated by the database.

penalty

  Returns a value indicating the "cost" of inserting the new entry into a particular branch of the tree. items will be inserted down the path of least `penalty` in the tree. Values returned by `penalty` should be non-negative. If a negative value is returned, it will be treated as zero.

picksplit

  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.

same

  Returns true if two entries are identical, false otherwise.

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