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Table 64.1. Built-inGiST Operator Classes

consistent

Given an index entryp and a query valueq, this function determines whether the index entry is“consistent” with the query; that is, could the predicate“indexed_columnindexable_operatorq” 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 istrue, arecheck flag must also be returned. This indicates whether the predicate is certainly true or only possibly true. Ifrecheck =false then the index has tested the predicate condition exactly, whereas ifrecheck =true the row is only a candidate match. In that case the system will automatically evaluate theindexable_operator against the actual row value to see if it is really a match. This convention allowsGiST to support both lossless and lossy index structures.

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_consistent);Datummy_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 andquery the value being looked up in the index. TheStrategyNumber parameter indicates which operator of your operator class is being applied — it matches one of the operator numbers in theCREATE OPERATOR CLASS command.

Depending on which operators you have included in the class, the data type ofquery 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 thequery argument value would have to depend on the operator.) It is recommended that the SQL declaration of theconsistent function use the opclass's indexed data type for thequery 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.

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_union);Datummy_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 whereunion(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 thisGiST support method.

The result of theunion 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). Theunion function should return a pointer to newlypalloc()ed memory. You can't just return the input value as-is, even if there is no type change.

As shown above, theunion function's firstinternal argument is actually aGistEntryVector pointer. The second argument is a pointer to an integer variable, which can be ignored. (It used to be required that theunion 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 thecompress method is omitted, data items are stored in the index without modification.

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_compress);Datummy_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 adaptcompressed_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 thedecompress 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 thecompress method; in particular, ifcompress is lossy then it's impossible fordecompress to exactly reconstruct the original data.decompress is not necessarily equivalent tofetch, either, since the other GiST methods might not require full reconstruction of the data.)

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_decompress);Datummy_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 leastpenalty in the tree. Values returned bypenalty should be non-negative. If a negative value is returned, it will be treated as zero.

TheSQL declaration of the function must look like this:

CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)RETURNS internalAS '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);Datummy_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, thepenalty function doesn't just return afloat 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.

Thepenalty 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.

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_picksplit);Datummy_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 thepicksplit function's result is delivered by modifying the passed-inv structure. The return value per se is ignored, though it's conventional to pass back the address ofv.

Likepenalty, thepicksplit function is crucial to good performance of the index. Designing suitablepenalty andpicksplit implementations is where the challenge of implementing well-performingGiST 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.)

TheSQL declaration of the function must look like this:

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

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

PG_FUNCTION_INFO_V1(my_same);Datummy_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, thesame 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 entryp and a query valueq, 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.

TheSQL declaration of the function must look like this:

CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)RETURNS float8AS 'MODULE_PATHNAME'LANGUAGE C STRICT;

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

PG_FUNCTION_INFO_V1(my_distance);Datummy_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 thedistance function are identical to the arguments of theconsistent 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 befloat8 orfloat4, 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 finitefloat8 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 thatdistance 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.

TheSQL declaration of the function must look like this:

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

The argument is a pointer to aGISTENTRY struct. On entry, itskey field contains a non-NULL leaf datum in compressed form. The return value is anotherGISTENTRY struct, whosekey field contains the same datum in its original, uncompressed form. If the opclass's compress function does nothing for leaf entries, thefetch method can return the argument as-is. Or, if the opclass does not have a compress function, thefetch 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);Datummy_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 afetch function.

options

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

TheSQL declaration of the function must look like this:

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

The function is passed a pointer to alocal_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 thePG_HAS_OPCLASS_OPTIONS() andPG_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 voidvalidate_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 Sizefill_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);Datummy_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);Datummy_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 inGiST is flexible, it may depend on user-specified parameters. For instance, the length of key signature may be specified. Seegtsvector_options() for example.

sortsupport

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

Thesortsupport method is optional. If it is not provided,CREATE INDEX builds the index by inserting each tuple to the tree using thepenalty andpicksplit functions, which is much slower.

TheSQL declaration of the function must look like this:

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

The argument is a pointer to aSortSupport 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 theSortSupport 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 thecompress method. The full API is defined insrc/include/utils/sortsupport.h.

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

PG_FUNCTION_INFO_V1(my_sortsupport);static intmy_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;}Datummy_sortsupport(PG_FUNCTION_ARGS){  SortSupport ssup = (SortSupport) PG_GETARG_POINTER(0);  ssup->comparator = my_fastcmp;  PG_RETURN_VOID();}

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 andbuffered modes.

The sorted method is only available if each of the opclasses used by the index provides asortsupport function, as described inSection 64.2.3. 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 thepenalty 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 reacheseffective_cache_size. Buffering can be manually forced or prevented by thebuffering 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.

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”