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 a few 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.
There are two methods that an operator class for GIN must provide:
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 38.15.2).
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 allows
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 bools 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.
An operator class must also provide a function to check if an indexed item
matches the query. It comes in two flavors, a boolean consistent
function, and a ternary triConsistent function.
triConsistent covers the functionality of both, so providing
triConsistent alone is sufficient. However, if the boolean
variant is significantly cheaper to calculate, it can be advantageous to
provide both. If only the boolean variant is provided, some optimizations
that depend on refuting index items before fetching all the keys are
disabled.
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 extractQuery for this query datum.
Each element of the
check array is true if the indexed item contains the
corresponding query key, i.e., if (check[i] == true) the i-th key of the
extractQuery result 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.
When 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
FROM. The consistent function 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.
GinTernaryValue triConsistent(GinTernaryValue check[], StrategyNumber n, Datum query,
int32 nkeys, Pointer extra_data[],
Datum queryKeys[], bool nullFlags[])
triConsistent is similar to consistent,
but instead of booleans in the check vector, there are
three possible values for each
key: GIN_TRUE, GIN_FALSE and
GIN_MAYBE. GIN_FALSE and GIN_TRUE
have the same meaning as regular boolean values, while
GIN_MAYBE means that the presence of that key is not known.
When GIN_MAYBE values are present, the function should only
return GIN_TRUE if the item certainly matches whether or
not the index item contains the corresponding query keys. Likewise, the
function must return GIN_FALSE only if the item certainly
does not match, whether or not it contains the GIN_MAYBE
keys. If the result depends on the GIN_MAYBE entries, i.e.,
the match cannot be confirmed or refuted based on the known query keys,
the function must return GIN_MAYBE.
When there are no GIN_MAYBE values in the check
vector, a GIN_MAYBE return value is the equivalent of
setting the recheck flag in the
boolean consistent function.
In addition, GIN must have a way to sort the key values stored in the index. The operator class can define the sort ordering by specifying a comparison method:
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.
Alternatively, if the operator class does not provide a compare
method, GIN will look up the default btree operator class for the index
key data type, and use its comparison function. It is recommended to
specify the comparison function in a GIN operator class that is meant for
just one data type, as looking up the btree operator class costs a few
cycles. However, polymorphic GIN operator classes (such
as array_ops) typically cannot specify a single comparison
function.
Optionally, an operator class for GIN can supply the following 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 function.
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 66.4.2 for details.
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 extractQuery,
consistent and triConsistent is whatever is the
right-hand input type of the class member operator identified by the
strategy number. This need not be the same as the indexed type, so long as
key values of the correct type can be extracted from it. However, it is
recommended that the SQL declarations of these three support functions use
the opclass's indexed data type for the query argument, even
though the actual type might be something else depending on the operator.