PostgreSQL 9.0.3 Documentation
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# 53.2. Extensibility

The GIN interface has a high level of abstraction, requiring the access method implementer only to implement the semantics of the data type being accessed. The GIN layer itself takes care of concurrency, logging and searching the tree structure.

All it takes to get a GIN 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, GIN combines extensibility with generality, code reuse, and a clean interface.

The four methods that an operator class for GIN must provide are:

`int compare(Datum a, Datum b)`

  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.

`Datum *extractValue(Datum inputValue, int32 *nkeys)`

  Returns an array of keys given a value to be indexed. The number of returned keys must be stored into `*nkeys`.

`Datum *extractQuery(Datum query, int32 *nkeys, StrategyNumber n, bool **pmatch, Pointer **extra_data)`

  Returns an array of keys given a value to be queried; that is, `query` is the value on the right-hand side of an indexable operator whose left-hand side is the indexed column. `n` is the strategy number of the operator within the operator class (see Section 35.14.2). Often, `extractQuery` will need to consult `n` to determine the data type of `query` and the key values that need to be extracted. The number of returned keys must be stored into `*nkeys`. If the query contains no keys then `extractQuery` should store 0 or -1 into `*nkeys`, depending on the semantics of the operator. 0 means that every value matches the `query` and a full-index scan should be performed (but see Section 53.5). -1 means that nothing can match the `query`, and so the index scan can be skipped entirely. `pmatch` is an output argument for use when partial match is supported. To use it, `extractQuery` must allocate an array of `*nkeys` Booleans and store its address at `*pmatch`. Each element of the array should be set to TRUE if the corresponding key requires partial match, FALSE if not. If `*pmatch` 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. `extra_data` is an output argument that allows `extractQuery` to pass additional data to the `consistent` and `comparePartial` methods. To use it, `extractQuery` must allocate an array of `*nkeys` Pointers and store its address at `*extra_data`, 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 `*extra_data` is set, the whole array is passed to the `consistent` method, and the appropriate element to the `comparePartial` method.

`bool consistent(bool check[], StrategyNumber n, Datum query, int32 nkeys, Pointer extra_data[], bool *recheck)`

  Returns TRUE if the indexed value satisfies the query operator with strategy number `n` (or might satisfy, if the recheck indication is returned). The `check` array has length `nkeys`, which is the same as the number of keys previously returned by `extractQuery` for this `query` datum. Each element of the `check` array is TRUE if the indexed value contains the corresponding query key, ie, if (check[i] == TRUE) the i-th key of the `extractQuery` result array is present in the indexed value. The original `query` datum (not the extracted key array!) is passed in case the `consistent` method needs to consult it. `extra_data` is the extra-data array returned by `extractQuery`, or NULL if none. On success, `*recheck` 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.

Optionally, an operator class for GIN can supply a fifth method:

`int comparePartial(Datum partial_key, Datum key, StrategyNumber n, Pointer extra_data)`

  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 `n` 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, `extra_data` is the corresponding element of the extra-data array made by `extractQuery`, or NULL if none.

To support "partial match" queries, an operator class must provide the `comparePartial` method, and its `extractQuery` method must set the `pmatch` parameter when a partial-match query is encountered. See Section 53.3.2 for details.

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