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 items, 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 two keys (not indexed items!) 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. Null keys are never passed to this function.
Datum *extractValue(Datum itemValue, int32 *nkeys, bool **nullFlags)
Returns a palloc'd array of keys given an item to be indexed. The number of returned keys must be stored into *nkeys. If any of the keys can be null, also palloc an array of *nkeys bool fields, store its address at *nullFlags, and set these null flags as needed. *nullFlags can be left NULL (its initial value) if all keys are non-null. The return value can be NULL if the item contains no keys.
Datum *extractQuery(Datum query, int32 *nkeys, StrategyNumber n, bool **pmatch, Pointer **extra_data, bool **nullFlags, int32 *searchMode)
Returns a palloc'd 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 method it should use to extract key
values. The number of returned keys must be stored into *nkeys. If any of the keys can be null, also palloc
an array of *nkeys bool fields, store its address at *nullFlags, and set these null flags as needed.
*nullFlags can be left NULL (its initial value) if all keys are non-null.
The return value can be NULL if the
query contains no keys.
searchMode is an output argument that
extractQuery to specify
details about how the search will be done. If *searchMode is set to GIN_SEARCH_MODE_DEFAULT (which is the value it is
initialized to before call), only items that match at least one of
the returned keys are considered candidate matches. If *searchMode is set to GIN_SEARCH_MODE_INCLUDE_EMPTY, then in addition to
items containing at least one matching key, items that contain no
keys at all are considered candidate matches. (This mode is useful
for implementing is-subset-of operators, for example.) If
*searchMode is set to GIN_SEARCH_MODE_ALL, then all non-null items in the
index are considered candidate matches, whether they match any of
the returned keys or not. (This mode is much slower than the other
two choices, since it requires scanning essentially the entire
index, but it may be necessary to implement corner cases correctly.
An operator that needs this mode in most cases is probably not a
good candidate for a GIN operator class.) The symbols to use for
setting this mode are defined in access/gin.h.
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
extractQuery to pass
additional data to the
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
method, and the appropriate element to the
bool consistent(bool check, StrategyNumber n, Datum query, int32 nkeys, Pointer extra_data, bool *recheck, Datum queryKeys, bool nullFlags)
Returns TRUE if an indexed item satisfies the query operator
with strategy number n (or might satisfy
it, if the recheck indication is returned). This function does not
have direct access to the indexed item's value, since
GIN does not store items
explicitly. Rather, what is available is knowledge about which key
values extracted from the query appear in a given indexed item. The
check array has length nkeys, which is the same as the number of keys
previously returned by
for this query datum. Each element of the
check array is TRUE if the indexed item
contains the corresponding query key, ie, if (check[i] == TRUE) the
i-th key of the
array is present in the indexed item. The original query datum is passed in case the
consistent method needs to consult it, and so are
the queryKeys and nullFlags arrays previously returned by
extractQuery. extra_data is the extra-data array returned by
extractQuery, or NULL if none.
extractQuery returns a null
key in queryKeys, the corresponding
check element is TRUE if the indexed
item contains a null key; that is, the semantics of check are like IS NOT DISTINCT
can examine the corresponding nullFlags
element if it needs to tell the difference between a regular value
match and a null match.
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. That is, a FALSE return value guarantees that the heap tuple does not match the query; a TRUE return value with *recheck set to FALSE guarantees that the heap tuple does match the query; and a TRUE return value with *recheck set to TRUE means that the heap tuple might match the query, so it needs to be fetched and rechecked by evaluating the query operator directly against the originally indexed item.
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 key 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. Null keys are never passed to this
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 57.3.2 for
The actual data types of the various Datum values mentioned above vary depending on the
operator class. The item values passed to
extractValue are always of the operator class's
input type, and all key values must be of the class's STORAGE type. The type of the query argument passed to
consistent is whatever is specified as the
right-hand input type of the class member operator identified by
the strategy number. This need not be the same as the item type, so
long as key values of the correct type can be extracted from