GiST Indexes {#gist}
============

index

GiST

Introduction {#gist-intro}
============

GiST stands for Generalized Search Tree. It is a balanced,
tree-structured access method, that acts as a base template in which to
implement arbitrary indexing schemes. B-trees, R-trees and many other
indexing schemes can be implemented in GiST.

One advantage of GiST is that it allows the development of custom data
types with the appropriate access methods, by an expert in the domain of
the data type, rather than a database expert.

Some of the information here is derived from the University of
California at Berkeley\'s GiST Indexing Project [web
site](http://gist.cs.berkeley.edu/) and Marcel Kornacker\'s thesis,
[Access Methods for Next-Generation Database
Systems](http://www.sai.msu.su/~megera/postgres/gist/papers/concurrency/access-methods-for-next-generation.pdf.gz).
The GiST implementation in PostgreSQL is primarily maintained by Teodor
Sigaev and Oleg Bartunov, and there is more information on their [web
site](http://www.sai.msu.su/~megera/postgres/gist/).

Built-in Operator Classes {#gist-builtin-opclasses}
=========================

The core PostgreSQL distribution includes the GiST operator classes
shown in [table\_title](#gist-builtin-opclasses-table). (Some of the
optional modules described in [???](#contrib) provide additional GiST
operator classes.)

  ----------------------------------------------------------------------------------------------------
  Name                                   Indexable Operators                  Ordering Operators
  -------------------------------------- ------------------------------------ ------------------------
  `box_ops`                              `<< (box, box)`                      `<-> (box, point)`

  `&< (box, box)`                                                             

  `&& (box, box)`                                                             

  `&> (box, box)`                                                             

  `>> (box, box)`                                                             

  `~= (box, box)`                                                             

  `@> (box, box)`                                                             

  `<@ (box, box)`                                                             

  `&<| (box, box)`                                                            

  `<<| (box, box)`                                                            

  `|>> (box, box)`                                                            

  `|&> (box, box)`                                                            

  `~ (box, box)`                                                              

  `@ (box, box)`                                                              

  `circle_ops`                           `<< (circle, circle)`                `<-> (circle, point)`

  `&< (circle, circle)`                                                       

  `&> (circle, circle)`                                                       

  `>> (circle, circle)`                                                       

  `<@ (circle, circle)`                                                       

  `@> (circle, circle)`                                                       

  `~= (circle, circle)`                                                       

  `&& (circle, circle)`                                                       

  `|>> (circle, circle)`                                                      

  `<<| (circle, circle)`                                                      

  `&<| (circle, circle)`                                                      

  `|&> (circle, circle)`                                                      

  `@ (circle, circle)`                                                        

  `~ (circle, circle)`                                                        

  `inet_ops`                             `<< (inet, inet)`                    

  `<<= (inet, inet)`                                                          

  `>> (inet, inet)`                                                           

  `>>= (inet, inet)`                                                          

  `= (inet, inet)`                                                            

  `<> (inet, inet)`                                                           

  `< (inet, inet)`                                                            

  `<= (inet, inet)`                                                           

  `> (inet, inet)`                                                            

  `>= (inet, inet)`                                                           

  `&& (inet, inet)`                                                           

  `multirange_ops`                       `= (anymultirange, anymultirange)`   

  `&& (anymultirange, anymultirange)`                                         

  `&& (anymultirange, anyrange)`                                              

  `@> (anymultirange, anyelement)`                                            

  `@> (anymultirange, anymultirange)`                                         

  `@> (anymultirange, anyrange)`                                              

  `<@ (anymultirange, anymultirange)`                                         

  `<@ (anymultirange, anyrange)`                                              

  `<< (anymultirange, anymultirange)`                                         

  `<< (anymultirange, anyrange)`                                              

  `>> (anymultirange, anymultirange)`                                         

  `>> (anymultirange, anyrange)`                                              

  `&< (anymultirange, anymultirange)`                                         

  `&< (anymultirange, anyrange)`                                              

  `&> (anymultirange, anymultirange)`                                         

  `&> (anymultirange, anyrange)`                                              

  `-|- (anymultirange, anymultirange)`                                        

  `-|- (anymultirange, anyrange)`                                             

  `point_ops`                            `|>> (point, point)`                 `<-> (point, point)`

  `<< (point, point)`                                                         

  `>> (point, point)`                                                         

  `<<| (point, point)`                                                        

  `~= (point, point)`                                                         

  `<@ (point, box)`                                                           

  `<@ (point, polygon)`                                                       

  `<@ (point, circle)`                                                        

  `poly_ops`                             `<< (polygon, polygon)`              `<-> (polygon, point)`

  `&< (polygon, polygon)`                                                     

  `&> (polygon, polygon)`                                                     

  `>> (polygon, polygon)`                                                     

  `<@ (polygon, polygon)`                                                     

  `@> (polygon, polygon)`                                                     

  `~= (polygon, polygon)`                                                     

  `&& (polygon, polygon)`                                                     

  `<<| (polygon, polygon)`                                                    

  `&<| (polygon, polygon)`                                                    

  `|&> (polygon, polygon)`                                                    

  `|>> (polygon, polygon)`                                                    

  `@ (polygon, polygon)`                                                      

  `~ (polygon, polygon)`                                                      

  `range_ops`                            `= (anyrange, anyrange)`             

  `&& (anyrange, anyrange)`                                                   

  `&& (anyrange, anymultirange)`                                              

  `@> (anyrange, anyelement)`                                                 

  `@> (anyrange, anyrange)`                                                   

  `@> (anyrange, anymultirange)`                                              

  `<@ (anyrange, anyrange)`                                                   

  `<@ (anyrange, anymultirange)`                                              

  `<< (anyrange, anyrange)`                                                   

  `<< (anyrange, anymultirange)`                                              

  `>> (anyrange, anyrange)`                                                   

  `>> (anyrange, anymultirange)`                                              

  `&< (anyrange, anyrange)`                                                   

  `&< (anyrange, anymultirange)`                                              

  `&> (anyrange, anyrange)`                                                   

  `&> (anyrange, anymultirange)`                                              

  `-|- (anyrange, anyrange)`                                                  

  `-|- (anyrange, anymultirange)`                                             

  `tsquery_ops`                          `<@ (tsquery, tsquery)`              

  `@> (tsquery, tsquery)`                                                     

  `tsvector_ops`                         `@@ (tsvector, tsquery)`             
  ----------------------------------------------------------------------------------------------------

  : Built-in GiST Operator Classes

For historical reasons, the `inet_ops` operator class is not the default
class for types `inet` and `cidr`. To use it, mention the class name in
`CREATE INDEX`, for example

    CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);

Extensibility {#gist-extensibility}
=============

Traditionally, implementing a new index access method meant a lot of
difficult work. It was necessary to understand the inner workings of the
database, such as the lock manager and Write-Ahead Log. The GiST
interface has a high level of abstraction, requiring the access method
implementer only to implement the semantics of the data type being
accessed. The GiST layer itself takes care of concurrency, logging and
searching the tree structure.

This extensibility should not be confused with the extensibility of the
other standard search trees in terms of the data they can handle. For
example, PostgreSQL supports extensible B-trees and hash indexes. That
means that you can use PostgreSQL to build a B-tree or hash over any
data type you want. But B-trees only support range predicates (`<`, `=`,
`>`), and hash indexes only support equality queries.

So if you index, say, an image collection with a PostgreSQL B-tree, you
can only issue queries such as "is imagex equal to imagey", "is imagex
less than imagey" and "is imagex greater than imagey". Depending on how
you define "equals", "less than" and "greater than" in this context,
this could be useful. However, by using a GiST based index, you could
create ways to ask domain-specific questions, perhaps "find all images
of horses" or "find all over-exposed images".

All it takes to get a GiST access method up and running is to implement
several user-defined methods, which define the behavior of keys in the
tree. Of course these methods have to be pretty fancy to support fancy
queries, but for all the standard queries (B-trees, R-trees, etc.)
they\'re relatively straightforward. In short, GiST combines
extensibility along with generality, code reuse, and a clean interface.

There are five methods that an index operator class for GiST must
provide, and six that are optional. Correctness of the index is ensured
by proper implementation of the `same`, `consistent` and `union`
methods, while efficiency (size and speed) of the index will depend on
the `penalty` and `picksplit` methods. Two optional methods are
`compress` and `decompress`, which allow an index to have internal tree
data of a different type than the data it indexes. The leaves are to be
of the indexed data type, while the other tree nodes can be of any C
struct (but you still have to follow PostgreSQL data type rules here,
see about `varlena` for variable sized data). If the tree\'s internal
data type exists at the SQL level, the `STORAGE` option of the
`CREATE OPERATOR CLASS` command can be used. The optional eighth method
is `distance`, which is needed if the operator class wishes to support
ordered scans (nearest-neighbor searches). The optional ninth method
`fetch` is needed if the operator class wishes to support index-only
scans, except when the `compress` method is omitted. The optional tenth
method `options` is needed if the operator class has user-specified
parameters. The optional eleventh method `sortsupport` is used to speed
up building a GiST index.

`consistent`

:   Given an index entry `p` and a query value `q`, this function
    determines whether the index entry is "consistent" with the query;
    that is, could the predicate "indexed\_column indexable\_operator
    `q`" be true for any row represented by the index entry? For a leaf
    index entry this is equivalent to testing the indexable condition,
    while for an internal tree node this determines whether it is
    necessary to scan the subtree of the index represented by the tree
    node. When the result is `true`, a `recheck` flag must also be
    returned. This indicates whether the predicate is certainly true or
    only possibly true. If `recheck` = `false` then the index has tested
    the predicate condition exactly, whereas if `recheck` = `true` the
    row is only a candidate match. In that case the system will
    automatically evaluate the indexable\_operator against the actual
    row value to see if it is really a match. This convention allows
    GiST to support both lossless and lossy index structures.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)
        RETURNS bool
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_consistent);

        Datum
        my_consistent(PG_FUNCTION_ARGS)
        {
            GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
            data_type  *query = PG_GETARG_DATA_TYPE_P(1);
            StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
            /* Oid subtype = PG_GETARG_OID(3); */
            bool       *recheck = (bool *) PG_GETARG_POINTER(4);
            data_type  *key = DatumGetDataType(entry->key);
            bool        retval;

            /*
             * determine return value as a function of strategy, key and query.
             *
             * Use GIST_LEAF(entry) to know where you're called in the index tree,
             * which comes handy when supporting the = operator for example (you could
             * check for non empty union() in non-leaf nodes and equality in leaf
             * nodes).
             */

            *recheck = true;        /* or false if check is exact */

            PG_RETURN_BOOL(retval);
        }

    Here, `key` is an element in the index and `query` the value being
    looked up in the index. The `StrategyNumber` parameter indicates
    which operator of your operator class is being applied MDASH it
    matches one of the operator numbers in the `CREATE OPERATOR CLASS`
    command.

    Depending on which operators you have included in the class, the
    data type of `query` could vary with the operator, since it will be
    whatever type is on the right-hand side of the operator, which might
    be different from the indexed data type appearing on the left-hand
    side. (The above code skeleton assumes that only one type is
    possible; if not, fetching the `query` argument value would have to
    depend on the operator.) It is recommended that the SQL declaration
    of the `consistent` function use the opclass\'s indexed data type
    for the `query` argument, even though the actual type might be
    something else depending on the operator.

`union`

:   This method consolidates information in the tree. Given a set of
    entries, this function generates a new index entry that represents
    all the given entries.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_union(internal, internal)
        RETURNS storage_type
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_union);

        Datum
        my_union(PG_FUNCTION_ARGS)
        {
            GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
            GISTENTRY  *ent = entryvec->vector;
            data_type  *out,
                       *tmp,
                       *old;
            int         numranges,
                        i = 0;

            numranges = entryvec->n;
            tmp = DatumGetDataType(ent[0].key);
            out = tmp;

            if (numranges == 1)
            {
                out = data_type_deep_copy(tmp);

                PG_RETURN_DATA_TYPE_P(out);
            }

            for (i = 1; i < numranges; i++)
            {
                old = out;
                tmp = DatumGetDataType(ent[i].key);
                out = my_union_implementation(out, tmp);
            }

            PG_RETURN_DATA_TYPE_P(out);
        }

    As you can see, in this skeleton we\'re dealing with a data type
    where `union(X, Y, Z) = union(union(X, Y), Z)`. It\'s easy enough to
    support data types where this is not the case, by implementing the
    proper union algorithm in this GiST support method.

    The result of the `union` function must be a value of the index\'s
    storage type, whatever that is (it might or might not be different
    from the indexed column\'s type). The `union` function should return
    a pointer to newly `palloc()`ed memory. You can\'t just return the
    input value as-is, even if there is no type change.

    As shown above, the `union` function\'s first `internal` argument is
    actually a GistEntryVector pointer. The second argument is a pointer
    to an integer variable, which can be ignored. (It used to be
    required that the `union` function store the size of its result
    value into that variable, but this is no longer necessary.)

`compress`

:   Converts a data item into a format suitable for physical storage in
    an index page. If the `compress` method is omitted, data items are
    stored in the index without modification.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_compress(internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_compress);

        Datum
        my_compress(PG_FUNCTION_ARGS)
        {
            GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
            GISTENTRY  *retval;

            if (entry->leafkey)
            {
                /* replace entry->key with a compressed version */
                compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type));

                /* fill *compressed_data from entry->key ... */

                retval = palloc(sizeof(GISTENTRY));
                gistentryinit(*retval, PointerGetDatum(compressed_data),
                              entry->rel, entry->page, entry->offset, FALSE);
            }
            else
            {
                /* typically we needn't do anything with non-leaf entries */
                retval = entry;
            }

            PG_RETURN_POINTER(retval);
        }

    You have to adapt compressed\_data\_type to the specific type
    you\'re converting to in order to compress your leaf nodes, of
    course.

`decompress`

:   Converts the stored representation of a data item into a format that
    can be manipulated by the other GiST methods in the operator class.
    If the `decompress` method is omitted, it is assumed that the other
    GiST methods can work directly on the stored data format.
    (`decompress` is not necessarily the reverse of the `compress`
    method; in particular, if `compress` is lossy then it\'s impossible
    for `decompress` to exactly reconstruct the original data.
    `decompress` is not necessarily equivalent to `fetch`, either, since
    the other GiST methods might not require full reconstruction of the
    data.)

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_decompress(internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_decompress);

        Datum
        my_decompress(PG_FUNCTION_ARGS)
        {
            PG_RETURN_POINTER(PG_GETARG_POINTER(0));
        }

    The above skeleton is suitable for the case where no decompression
    is needed. (But, of course, omitting the method altogether is even
    easier, and is recommended in such cases.)

`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.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;  -- in some cases penalty functions need not be strict

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_penalty);

        Datum
        my_penalty(PG_FUNCTION_ARGS)
        {
            GISTENTRY  *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
            GISTENTRY  *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
            float      *penalty = (float *) PG_GETARG_POINTER(2);
            data_type  *orig = DatumGetDataType(origentry->key);
            data_type  *new = DatumGetDataType(newentry->key);

            *penalty = my_penalty_implementation(orig, new);
            PG_RETURN_POINTER(penalty);
        }

    For historical reasons, the `penalty` function doesn\'t just return
    a `float` result; instead it has to store the value at the location
    indicated by the third argument. The return value per se is ignored,
    though it\'s conventional to pass back the address of that argument.

    The `penalty` function is crucial to good performance of the index.
    It\'ll get used at insertion time to determine which branch to
    follow when choosing where to add the new entry in the tree. At
    query time, the more balanced the index, the quicker the lookup.

`picksplit`

:   When an index 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.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_picksplit(internal, internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_picksplit);

        Datum
        my_picksplit(PG_FUNCTION_ARGS)
        {
            GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
            GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
            OffsetNumber maxoff = entryvec->n - 1;
            GISTENTRY  *ent = entryvec->vector;
            int         i,
                        nbytes;
            OffsetNumber *left,
                       *right;
            data_type  *tmp_union;
            data_type  *unionL;
            data_type  *unionR;
            GISTENTRY **raw_entryvec;

            maxoff = entryvec->n - 1;
            nbytes = (maxoff + 1) * sizeof(OffsetNumber);

            v->spl_left = (OffsetNumber *) palloc(nbytes);
            left = v->spl_left;
            v->spl_nleft = 0;

            v->spl_right = (OffsetNumber *) palloc(nbytes);
            right = v->spl_right;
            v->spl_nright = 0;

            unionL = NULL;
            unionR = NULL;

            /* Initialize the raw entry vector. */
            raw_entryvec = (GISTENTRY **) malloc(entryvec->n * sizeof(void *));
            for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
                raw_entryvec[i] = &(entryvec->vector[i]);

            for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
            {
                int         real_index = raw_entryvec[i] - entryvec->vector;

                tmp_union = DatumGetDataType(entryvec->vector[real_index].key);
                Assert(tmp_union != NULL);

                /*
                 * Choose where to put the index entries and update unionL and unionR
                 * accordingly. Append the entries to either v->spl_left or
                 * v->spl_right, and care about the counters.
                 */

                if (my_choice_is_left(unionL, curl, unionR, curr))
                {
                    if (unionL == NULL)
                        unionL = tmp_union;
                    else
                        unionL = my_union_implementation(unionL, tmp_union);

                    *left = real_index;
                    ++left;
                    ++(v->spl_nleft);
                }
                else
                {
                    /*
                     * Same on the right
                     */
                }
            }

            v->spl_ldatum = DataTypeGetDatum(unionL);
            v->spl_rdatum = DataTypeGetDatum(unionR);
            PG_RETURN_POINTER(v);
        }

    Notice that the `picksplit` function\'s result is delivered by
    modifying the passed-in v structure. The return value per se is
    ignored, though it\'s conventional to pass back the address of v.

    Like `penalty`, the `picksplit` function is crucial to good
    performance of the index. Designing suitable `penalty` and
    `picksplit` implementations is where the challenge of implementing
    well-performing GiST indexes lies.

`same`

:   Returns true if two index entries are identical, false otherwise.
    (An "index entry" is a value of the index\'s storage type, not
    necessarily the original indexed column\'s type.)

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_same);

        Datum
        my_same(PG_FUNCTION_ARGS)
        {
            prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
            prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
            bool       *result = (bool *) PG_GETARG_POINTER(2);

            *result = my_eq(v1, v2);
            PG_RETURN_POINTER(result);
        }

    For historical reasons, the `same` function doesn\'t just return a
    Boolean result; instead it has to store the flag at the location
    indicated by the third argument. The return value per se is ignored,
    though it\'s conventional to pass back the address of that argument.

`distance`

:   Given an index entry `p` and a query value `q`, this function
    determines the index entry\'s "distance" from the query value. This
    function must be supplied if the operator class contains any
    ordering operators. A query using the ordering operator will be
    implemented by returning index entries with the smallest "distance"
    values first, so the results must be consistent with the operator\'s
    semantics. For a leaf index entry the result just represents the
    distance to the index entry; for an internal tree node, the result
    must be the smallest distance that any child entry could have.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)
        RETURNS float8
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    And the matching code in the C module could then follow this
    skeleton:

        PG_FUNCTION_INFO_V1(my_distance);

        Datum
        my_distance(PG_FUNCTION_ARGS)
        {
            GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
            data_type  *query = PG_GETARG_DATA_TYPE_P(1);
            StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
            /* Oid subtype = PG_GETARG_OID(3); */
            /* bool *recheck = (bool *) PG_GETARG_POINTER(4); */
            data_type  *key = DatumGetDataType(entry->key);
            double      retval;

            /*
             * determine return value as a function of strategy, key and query.
             */

            PG_RETURN_FLOAT8(retval);
        }

    The arguments to the `distance` function are identical to the
    arguments of the `consistent` function.

    Some approximation is allowed when determining the distance, so long
    as the result is never greater than the entry\'s actual distance.
    Thus, for example, distance to a bounding box is usually sufficient
    in geometric applications. For an internal tree node, the distance
    returned must not be greater than the distance to any of the child
    nodes. If the returned distance is not exact, the function must set
    `*recheck` to true. (This is not necessary for internal tree nodes;
    for them, the calculation is always assumed to be inexact.) In this
    case the executor will calculate the accurate distance after
    fetching the tuple from the heap, and reorder the tuples if
    necessary.

    If the distance function returns `*recheck = true` for any leaf
    node, the original ordering operator\'s return type must be `float8`
    or `float4`, and the distance function\'s result values must be
    comparable to those of the original ordering operator, since the
    executor will sort using both distance function results and
    recalculated ordering-operator results. Otherwise, the distance
    function\'s result values can be any finite `float8` values, so long
    as the relative order of the result values matches the order
    returned by the ordering operator. (Infinity and minus infinity are
    used internally to handle cases such as nulls, so it is not
    recommended that `distance` functions return these values.)

`fetch`

:   Converts the compressed index representation of a data item into the
    original data type, for index-only scans. The returned data must be
    an exact, non-lossy copy of the originally indexed value.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_fetch(internal)
        RETURNS internal
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    The argument is a pointer to a GISTENTRY struct. On entry, its key
    field contains a non-NULL leaf datum in compressed form. The return
    value is another GISTENTRY struct, whose key field contains the same
    datum in its original, uncompressed form. If the opclass\'s compress
    function does nothing for leaf entries, the `fetch` method can
    return the argument as-is. Or, if the opclass does not have a
    compress function, the `fetch` method can be omitted as well, since
    it would necessarily be a no-op.

    The matching code in the C module could then follow this skeleton:

        PG_FUNCTION_INFO_V1(my_fetch);

        Datum
        my_fetch(PG_FUNCTION_ARGS)
        {
            GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
            input_data_type *in = DatumGetPointer(entry->key);
            fetched_data_type *fetched_data;
            GISTENTRY  *retval;

            retval = palloc(sizeof(GISTENTRY));
            fetched_data = palloc(sizeof(fetched_data_type));

            /*
             * Convert 'fetched_data' into the a Datum of the original datatype.
             */

            /* fill *retval from fetched_data. */
            gistentryinit(*retval, PointerGetDatum(converted_datum),
                          entry->rel, entry->page, entry->offset, FALSE);

            PG_RETURN_POINTER(retval);
        }

    If the compress method is lossy for leaf entries, the operator class
    cannot support index-only scans, and must not define a `fetch`
    function.

`options`

:   Allows definition of user-visible parameters that control operator
    class behavior.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_options(internal)
        RETURNS void
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    The function is passed a pointer to a local\_relopts struct, which
    needs to be filled with a set of operator class specific options.
    The options can be accessed from other support functions using the
    `PG_HAS_OPCLASS_OPTIONS()` and `PG_GET_OPCLASS_OPTIONS()` macros.

    An example implementation of my\_options() and parameters use from
    other support functions are given below:

        typedef enum MyEnumType
        {
            MY_ENUM_ON,
            MY_ENUM_OFF,
            MY_ENUM_AUTO
        } MyEnumType;

        typedef struct
        {
            int32   vl_len_;    /* varlena header (do not touch directly!) */
            int     int_param;  /* integer parameter */
            double  real_param; /* real parameter */
            MyEnumType enum_param; /* enum parameter */
            int     str_param;  /* string parameter */
        } MyOptionsStruct;

        /* String representation of enum values */
        static relopt_enum_elt_def myEnumValues[] =
        {
            {"on", MY_ENUM_ON},
            {"off", MY_ENUM_OFF},
            {"auto", MY_ENUM_AUTO},
            {(const char *) NULL}   /* list terminator */
        };

        static char *str_param_default = "default";

        /*
         * Sample validator: checks that string is not longer than 8 bytes.
         */
        static void
        validate_my_string_relopt(const char *value)
        {
            if (strlen(value) > 8)
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("str_param must be at most 8 bytes")));
        }

        /*
         * Sample filler: switches characters to lower case.
         */
        static Size
        fill_my_string_relopt(const char *value, void *ptr)
        {
            char   *tmp = str_tolower(value, strlen(value), DEFAULT_COLLATION_OID);
            int     len = strlen(tmp);

            if (ptr)
                strcpy((char *) ptr, tmp);

            pfree(tmp);
            return len + 1;
        }

        PG_FUNCTION_INFO_V1(my_options);

        Datum
        my_options(PG_FUNCTION_ARGS)
        {
            local_relopts *relopts = (local_relopts *) PG_GETARG_POINTER(0);

            init_local_reloptions(relopts, sizeof(MyOptionsStruct));
            add_local_int_reloption(relopts, "int_param", "integer parameter",
                                    100, 0, 1000000,
                                    offsetof(MyOptionsStruct, int_param));
            add_local_real_reloption(relopts, "real_param", "real parameter",
                                     1.0, 0.0, 1000000.0,
                                     offsetof(MyOptionsStruct, real_param));
            add_local_enum_reloption(relopts, "enum_param", "enum parameter",
                                     myEnumValues, MY_ENUM_ON,
                                     "Valid values are: \"on\", \"off\" and \"auto\".",
                                     offsetof(MyOptionsStruct, enum_param));
            add_local_string_reloption(relopts, "str_param", "string parameter",
                                       str_param_default,
                                       &validate_my_string_relopt,
                                       &fill_my_string_relopt,
                                       offsetof(MyOptionsStruct, str_param));

            PG_RETURN_VOID();
        }

        PG_FUNCTION_INFO_V1(my_compress);

        Datum
        my_compress(PG_FUNCTION_ARGS)
        {
            int     int_param = 100;
            double  real_param = 1.0;
            MyEnumType enum_param = MY_ENUM_ON;
            char   *str_param = str_param_default;

            /*
             * Normally, when opclass contains 'options' method, then options are always
             * passed to support functions.  However, if you add 'options' method to
             * existing opclass, previously defined indexes have no options, so the
             * check is required.
             */
            if (PG_HAS_OPCLASS_OPTIONS())
            {
                MyOptionsStruct *options = (MyOptionsStruct *) PG_GET_OPCLASS_OPTIONS();

                int_param = options->int_param;
                real_param = options->real_param;
                enum_param = options->enum_param;
                str_param = GET_STRING_RELOPTION(options, str_param);
            }

            /* the rest implementation of support function */
        }

    Since the representation of the key in GiST is flexible, it may
    depend on user-specified parameters. For instance, the length of key
    signature may be specified. See `gtsvector_options()` for example.

`sortsupport`

:   Returns a comparator function to sort data in a way that preserves
    locality. It is used by `CREATE INDEX` and `REINDEX` commands. The
    quality of the created index depends on how well the sort order
    determined by the comparator function preserves locality of the
    inputs.

    The `sortsupport` method is optional. If it is not provided,
    `CREATE INDEX` builds the index by inserting each tuple to the tree
    using the `penalty` and `picksplit` functions, which is much slower.

    The SQL declaration of the function must look like this:

        CREATE OR REPLACE FUNCTION my_sortsupport(internal)
        RETURNS void
        AS 'MODULE_PATHNAME'
        LANGUAGE C STRICT;

    The argument is a pointer to a SortSupport struct. At a minimum, the
    function must fill in its comparator field. The comparator takes
    three arguments: two Datums to compare, and a pointer to the
    SortSupport struct. The Datums are the two indexed values in the
    format that they are stored in the index; that is, in the format
    returned by the `compress` method. The full API is defined in
    `src/include/utils/sortsupport.h`.

    The matching code in the C module could then follow this skeleton:

        PG_FUNCTION_INFO_V1(my_sortsupport);

        static int
        my_fastcmp(Datum x, Datum y, SortSupport ssup)
        {
          /* establish order between x and y by computing some sorting value z */

          int z1 = ComputeSpatialCode(x);
          int z2 = ComputeSpatialCode(y);

          return z1 == z2 ? 0 : z1 > z2 ? 1 : -1;
        }

        Datum
        my_sortsupport(PG_FUNCTION_ARGS)
        {
          SortSupport ssup = (SortSupport) PG_GETARG_POINTER(0);

          ssup->comparator = my_fastcmp;
          PG_RETURN_VOID();
        }

All the GiST support methods are normally called in short-lived memory
contexts; that is, `CurrentMemoryContext` will get reset after each
tuple is processed. It is therefore not very important to worry about
pfree\'ing everything you palloc. However, in some cases it\'s useful
for a support method to cache data across repeated calls. To do that,
allocate the longer-lived data in `fcinfo->flinfo->fn_mcxt`, and keep a
pointer to it in `fcinfo->flinfo->fn_extra`. Such data will survive for
the life of the index operation (e.g., a single GiST index scan, index
build, or index tuple insertion). Be careful to pfree the previous value
when replacing a `fn_extra` value, or the leak will accumulate for the
duration of the operation.

Implementation {#gist-implementation}
==============

GiST Index Build Methods {#gist-buffering-build}
------------------------

The simplest way to build a GiST index is just to insert all the
entries, one by one. This tends to be slow for large indexes, because if
the index tuples are scattered across the index and the index is large
enough to not fit in cache, a lot of random I/O will be needed.
PostgreSQL supports two alternative methods for initial build of a GiST
index: sorted and buffered modes.

The sorted method is only available if each of the opclasses used by the
index provides a `sortsupport` function, as described in
[Extensibility](#gist-extensibility). If they do, this method is usually
the best, so it is used by default.

The buffered method works by not inserting tuples directly into the
index right away. It can dramatically reduce the amount of random I/O
needed for non-ordered data sets. For well-ordered data sets the benefit
is smaller or non-existent, because only a small number of pages receive
new tuples at a time, and those pages fit in cache even if the index as
a whole does not.

The buffered method needs to call the `penalty` function more often than
the simple method does, which consumes some extra CPU resources. Also,
the buffers need temporary disk space, up to the size of the resulting
index. Buffering can also influence the quality of the resulting index,
in both positive and negative directions. That influence depends on
various factors, like the distribution of the input data and the
operator class implementation.

If sorting is not possible, then by default a GiST index build switches
to the buffering method when the index size reaches
[???](#guc-effective-cache-size). Buffering can be manually forced or
prevented by the `buffering` parameter to the CREATE INDEX command. The
default behavior is good for most cases, but turning buffering off might
speed up the build somewhat if the input data is ordered.

Examples {#gist-examples}
========

The PostgreSQL source distribution includes several examples of index
methods implemented using GiST. The core system currently provides text
search support (indexing for `tsvector` and `tsquery`) as well as R-Tree
equivalent functionality for some of the built-in geometric data types
(see `src/backend/access/gist/gistproc.c`). The following `contrib`
modules also contain GiST operator classes:

`btree_gist`

:   B-tree equivalent functionality for several data types

`cube`

:   Indexing for multidimensional cubes

`hstore`

:   Module for storing (key, value) pairs

`intarray`

:   RD-Tree for one-dimensional array of int4 values

`ltree`

:   Indexing for tree-like structures

`pg_trgm`

:   Text similarity using trigram matching

`seg`

:   Indexing for "float ranges"
