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52.2. 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 34.14.2). 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 52.5). -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 52.3.2 for details.

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