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54.2. Extensibility

SP-GiST offers an interface with a high level of abstraction, requiring the access method developer to implement only methods specific to a given data type. The SP-GiST core is responsible for efficient disk mapping and searching the tree structure. It also takes care of concurrency and logging considerations.

Leaf tuples of an SP-GiST tree contain values of the same data type as the indexed column. Leaf tuples at the root level will always contain the original indexed data value, but leaf tuples at lower levels might contain only a compressed representation, such as a suffix. In that case the operator class support functions must be able to reconstruct the original value using information accumulated from the inner tuples that are passed through to reach the leaf level.

Inner tuples are more complex, since they are branching points in the search tree. Each inner tuple contains a set of one or more nodes, which represent groups of similar leaf values. A node contains a downlink that leads to either another, lower-level inner tuple, or a short list of leaf tuples that all lie on the same index page. Each node has a label that describes it; for example, in a suffix tree the node label could be the next character of the string value. Optionally, an inner tuple can have a prefix value that describes all its members. In a suffix tree this could be the common prefix of the represented strings. The prefix value is not necessarily really a prefix, but can be any data needed by the operator class; for example, in a quad-tree it can store the central point that the four quadrants are measured with respect to. A quad-tree inner tuple would then also contain four nodes corresponding to the quadrants around this central point.

Some tree algorithms require knowledge of level (or depth) of the current tuple, so the SP-GiST core provides the possibility for operator classes to manage level counting while descending the tree. There is also support for incrementally reconstructing the represented value when that is needed.

Note: The SP-GiST core code takes care of null entries. Although SP-GiST indexes do store entries for nulls in indexed columns, this is hidden from the index operator class code: no null index entries or search conditions will ever be passed to the operator class methods. (It is assumed that SP-GiST operators are strict and so cannot succeed for null values.) Null values are therefore not discussed further here.

There are five user-defined methods that an index operator class for SP-GiST must provide. All five follow the convention of accepting two internal arguments, the first of which is a pointer to a C struct containing input values for the support method, while the second argument is a pointer to a C struct where output values must be placed. Four of the methods just return void, since all their results appear in the output struct; but leaf_consistent additionally returns a boolean result. The methods must not modify any fields of their input structs. In all cases, the output struct is initialized to zeroes before calling the user-defined method.

The five user-defined methods are:

config

Returns static information about the index implementation, including the data type OIDs of the prefix and node label data types.

The SQL declaration of the function must look like this:

CREATE FUNCTION my_config(internal, internal) RETURNS void ...

The first argument is a pointer to a spgConfigIn C struct, containing input data for the function. The second argument is a pointer to a spgConfigOut C struct, which the function must fill with result data.

typedef struct spgConfigIn
{
    Oid         attType;        /* Data type to be indexed */
} spgConfigIn;

typedef struct spgConfigOut
{
    Oid         prefixType;     /* Data type of inner-tuple prefixes */
    Oid         labelType;      /* Data type of inner-tuple node labels */
    bool        canReturnData;  /* Opclass can reconstruct original data */
    bool        longValuesOK;   /* Opclass can cope with values > 1 page */
} spgConfigOut;

attType is passed in order to support polymorphic index operator classes; for ordinary fixed-data-type operator classes, it will always have the same value and so can be ignored.

For operator classes that do not use prefixes, prefixType can be set to VOIDOID. Likewise, for operator classes that do not use node labels, labelType can be set to VOIDOID. canReturnData should be set true if the operator class is capable of reconstructing the originally-supplied index value. longValuesOK should be set true only when the attType is of variable length and the operator class is capable of segmenting long values by repeated suffixing (see Section 54.3.1).

choose

Chooses a method for inserting a new value into an inner tuple.

The SQL declaration of the function must look like this:

CREATE FUNCTION my_choose(internal, internal) RETURNS void ...

The first argument is a pointer to a spgChooseIn C struct, containing input data for the function. The second argument is a pointer to a spgChooseOut C struct, which the function must fill with result data.

typedef struct spgChooseIn
{
    Datum       datum;          /* original datum to be indexed */
    Datum       leafDatum;      /* current datum to be stored at leaf */
    int         level;          /* current level (counting from zero) */

    /* Data from current inner tuple */
    bool        allTheSame;     /* tuple is marked all-the-same? */
    bool        hasPrefix;      /* tuple has a prefix? */
    Datum       prefixDatum;    /* if so, the prefix value */
    int         nNodes;         /* number of nodes in the inner tuple */
    Datum      *nodeLabels;     /* node label values (NULL if none) */
} spgChooseIn;

typedef enum spgChooseResultType
{
    spgMatchNode = 1,           /* descend into existing node */
    spgAddNode,                 /* add a node to the inner tuple */
    spgSplitTuple               /* split inner tuple (change its prefix) */
} spgChooseResultType;

typedef struct spgChooseOut
{
    spgChooseResultType resultType;     /* action code, see above */
    union
    {
        struct                  /* results for spgMatchNode */
        {
            int         nodeN;      /* descend to this node (index from 0) */
            int         levelAdd;   /* increment level by this much */
            Datum       restDatum;  /* new leaf datum */
        }           matchNode;
        struct                  /* results for spgAddNode */
        {
            Datum       nodeLabel;  /* new node's label */
            int         nodeN;      /* where to insert it (index from 0) */
        }           addNode;
        struct                  /* results for spgSplitTuple */
        {
            /* Info to form new inner tuple with one node */
            bool        prefixHasPrefix;    /* tuple should have a prefix? */
            Datum       prefixPrefixDatum;  /* if so, its value */
            Datum       nodeLabel;          /* node's label */

            /* Info to form new lower-level inner tuple with all old nodes */
            bool        postfixHasPrefix;   /* tuple should have a prefix? */
            Datum       postfixPrefixDatum; /* if so, its value */
        }           splitTuple;
    }           result;
} spgChooseOut;

datum is the original datum that was to be inserted into the index. leafDatum is initially the same as datum, but can change at lower levels of the tree if the choose or picksplit methods change it. When the insertion search reaches a leaf page, the current value of leafDatum is what will be stored in the newly created leaf tuple. level is the current inner tuple's level, starting at zero for the root level. allTheSame is true if the current inner tuple is marked as containing multiple equivalent nodes (see Section 54.3.3). hasPrefix is true if the current inner tuple contains a prefix; if so, prefixDatum is its value. nNodes is the number of child nodes contained in the inner tuple, and nodeLabels is an array of their label values, or NULL if there are no labels.

The choose function can determine either that the new value matches one of the existing child nodes, or that a new child node must be added, or that the new value is inconsistent with the tuple prefix and so the inner tuple must be split to create a less restrictive prefix.

If the new value matches one of the existing child nodes, set resultType to spgMatchNode. Set nodeN to the index (from zero) of that node in the node array. Set levelAdd to the increment in level caused by descending through that node, or leave it as zero if the operator class does not use levels. Set restDatum to equal datum if the operator class does not modify datums from one level to the next, or otherwise set it to the modified value to be used as leafDatum at the next level.

If a new child node must be added, set resultType to spgAddNode. Set nodeLabel to the label to be used for the new node, and set nodeN to the index (from zero) at which to insert the node in the node array. After the node has been added, the choose function will be called again with the modified inner tuple; that call should result in an spgMatchNode result.

If the new value is inconsistent with the tuple prefix, set resultType to spgSplitTuple. This action moves all the existing nodes into a new lower-level inner tuple, and replaces the existing inner tuple with a tuple having a single node that links to the new lower-level inner tuple. Set prefixHasPrefix to indicate whether the new upper tuple should have a prefix, and if so set prefixPrefixDatum to the prefix value. This new prefix value must be sufficiently less restrictive than the original to accept the new value to be indexed, and it should be no longer than the original prefix. Set nodeLabel to the label to be used for the node that will point to the new lower-level inner tuple. Set postfixHasPrefix to indicate whether the new lower-level inner tuple should have a prefix, and if so set postfixPrefixDatum to the prefix value. The combination of these two prefixes and the additional label must have the same meaning as the original prefix, because there is no opportunity to alter the node labels that are moved to the new lower-level tuple, nor to change any child index entries. After the node has been split, the choose function will be called again with the replacement inner tuple. That call will usually result in an spgAddNode result, since presumably the node label added in the split step will not match the new value; so after that, there will be a third call that finally returns spgMatchNode and allows the insertion to descend to the leaf level.

picksplit

Decides how to create a new inner tuple over a set of leaf tuples.

The SQL declaration of the function must look like this:

CREATE FUNCTION my_picksplit(internal, internal) RETURNS void ...

The first argument is a pointer to a spgPickSplitIn C struct, containing input data for the function. The second argument is a pointer to a spgPickSplitOut C struct, which the function must fill with result data.

typedef struct spgPickSplitIn
{
    int         nTuples;        /* number of leaf tuples */
    Datum      *datums;         /* their datums (array of length nTuples) */
    int         level;          /* current level (counting from zero) */
} spgPickSplitIn;

typedef struct spgPickSplitOut
{
    bool        hasPrefix;      /* new inner tuple should have a prefix? */
    Datum       prefixDatum;    /* if so, its value */

    int         nNodes;         /* number of nodes for new inner tuple */
    Datum      *nodeLabels;     /* their labels (or NULL for no labels) */

    int        *mapTuplesToNodes;   /* node index for each leaf tuple */
    Datum      *leafTupleDatums;    /* datum to store in each new leaf tuple */
} spgPickSplitOut;

nTuples is the number of leaf tuples provided. datums is an array of their datum values. level is the current level that all the leaf tuples share, which will become the level of the new inner tuple.

Set hasPrefix to indicate whether the new inner tuple should have a prefix, and if so set prefixDatum to the prefix value. Set nNodes to indicate the number of nodes that the new inner tuple will contain, and set nodeLabels to an array of their label values. (If the nodes do not require labels, set nodeLabels to NULL; see Section 54.3.2 for details.) Set mapTuplesToNodes to an array that gives the index (from zero) of the node that each leaf tuple should be assigned to. Set leafTupleDatums to an array of the values to be stored in the new leaf tuples (these will be the same as the input datums if the operator class does not modify datums from one level to the next). Note that the picksplit function is responsible for palloc'ing the nodeLabels, mapTuplesToNodes and leafTupleDatums arrays.

If more than one leaf tuple is supplied, it is expected that the picksplit function will classify them into more than one node; otherwise it is not possible to split the leaf tuples across multiple pages, which is the ultimate purpose of this operation. Therefore, if the picksplit function ends up placing all the leaf tuples in the same node, the core SP-GiST code will override that decision and generate an inner tuple in which the leaf tuples are assigned at random to several identically-labeled nodes. Such a tuple is marked allTheSame to signify that this has happened. The choose and inner_consistent functions must take suitable care with such inner tuples. See Section 54.3.3 for more information.

picksplit can be applied to a single leaf tuple only in the case that the config function set longValuesOK to true and a larger-than-a-page input value has been supplied. In this case the point of the operation is to strip off a prefix and produce a new, shorter leaf datum value. The call will be repeated until a leaf datum short enough to fit on a page has been produced. See Section 54.3.1 for more information.

inner_consistent

Returns set of nodes (branches) to follow during tree search.

The SQL declaration of the function must look like this:

CREATE FUNCTION my_inner_consistent(internal, internal) RETURNS void ...

The first argument is a pointer to a spgInnerConsistentIn C struct, containing input data for the function. The second argument is a pointer to a spgInnerConsistentOut C struct, which the function must fill with result data.

typedef struct spgInnerConsistentIn
{
    ScanKey     scankeys;       /* array of operators and comparison values */
    int         nkeys;          /* length of array */

    Datum       reconstructedValue;     /* value reconstructed at parent */
    int         level;          /* current level (counting from zero) */
    bool        returnData;     /* original data must be returned? */

    /* Data from current inner tuple */
    bool        allTheSame;     /* tuple is marked all-the-same? */
    bool        hasPrefix;      /* tuple has a prefix? */
    Datum       prefixDatum;    /* if so, the prefix value */
    int         nNodes;         /* number of nodes in the inner tuple */
    Datum      *nodeLabels;     /* node label values (NULL if none) */
} spgInnerConsistentIn;

typedef struct spgInnerConsistentOut
{
    int         nNodes;         /* number of child nodes to be visited */
    int        *nodeNumbers;    /* their indexes in the node array */
    int        *levelAdds;      /* increment level by this much for each */
    Datum      *reconstructedValues;    /* associated reconstructed values */
} spgInnerConsistentOut;

The array scankeys, of length nkeys, describes the index search condition(s). These conditions are combined with AND — only index entries that satisfy all of them are interesting. (Note that nkeys = 0 implies that all index entries satisfy the query.) Usually the consistent function only cares about the sk_strategy and sk_argument fields of each array entry, which respectively give the indexable operator and comparison value. In particular it is not necessary to check sk_flags to see if the comparison value is NULL, because the SP-GiST core code will filter out such conditions. reconstructedValue is the value reconstructed for the parent tuple; it is (Datum) 0 at the root level or if the inner_consistent function did not provide a value at the parent level. level is the current inner tuple's level, starting at zero for the root level. returnData is true if reconstructed data is required for this query; this will only be so if the config function asserted canReturnData. allTheSame is true if the current inner tuple is marked "all-the-same"; in this case all the nodes have the same label (if any) and so either all or none of them match the query (see Section 54.3.3). hasPrefix is true if the current inner tuple contains a prefix; if so, prefixDatum is its value. nNodes is the number of child nodes contained in the inner tuple, and nodeLabels is an array of their label values, or NULL if the nodes do not have labels.

nNodes must be set to the number of child nodes that need to be visited by the search, and nodeNumbers must be set to an array of their indexes. If the operator class keeps track of levels, set levelAdds to an array of the level increments required when descending to each node to be visited. (Often these increments will be the same for all the nodes, but that's not necessarily so, so an array is used.) If value reconstruction is needed, set reconstructedValues to an array of the values reconstructed for each child node to be visited; otherwise, leave reconstructedValues as NULL. Note that the inner_consistent function is responsible for palloc'ing the nodeNumbers, levelAdds and reconstructedValues arrays.

leaf_consistent

Returns true if a leaf tuple satisfies a query.

The SQL declaration of the function must look like this:

CREATE FUNCTION my_leaf_consistent(internal, internal) RETURNS bool ...

The first argument is a pointer to a spgLeafConsistentIn C struct, containing input data for the function. The second argument is a pointer to a spgLeafConsistentOut C struct, which the function must fill with result data.

typedef struct spgLeafConsistentIn
{
    ScanKey     scankeys;       /* array of operators and comparison values */
    int         nkeys;          /* length of array */

    Datum       reconstructedValue;     /* value reconstructed at parent */
    int         level;          /* current level (counting from zero) */
    bool        returnData;     /* original data must be returned? */

    Datum       leafDatum;      /* datum in leaf tuple */
} spgLeafConsistentIn;

typedef struct spgLeafConsistentOut
{
    Datum       leafValue;      /* reconstructed original data, if any */
    bool        recheck;        /* set true if operator must be rechecked */
} spgLeafConsistentOut;

The array scankeys, of length nkeys, describes the index search condition(s). These conditions are combined with AND — only index entries that satisfy all of them satisfy the query. (Note that nkeys = 0 implies that all index entries satisfy the query.) Usually the consistent function only cares about the sk_strategy and sk_argument fields of each array entry, which respectively give the indexable operator and comparison value. In particular it is not necessary to check sk_flags to see if the comparison value is NULL, because the SP-GiST core code will filter out such conditions. reconstructedValue is the value reconstructed for the parent tuple; it is (Datum) 0 at the root level or if the inner_consistent function did not provide a value at the parent level. level is the current leaf tuple's level, starting at zero for the root level. returnData is true if reconstructed data is required for this query; this will only be so if the config function asserted canReturnData. leafDatum is the key value stored in the current leaf tuple.

The function must return true if the leaf tuple matches the query, or false if not. In the true case, if returnData is true then leafValue must be set to the value originally supplied to be indexed for this leaf tuple. Also, recheck may be set to true if the match is uncertain and so the operator(s) must be re-applied to the actual heap tuple to verify the match.

All the SP-GiST support methods are normally called in a short-lived memory context; that is, CurrentMemoryContext will be reset after processing of each tuple. It is therefore not very important to worry about pfree'ing everything you palloc. (The config method is an exception: it should try to avoid leaking memory. But usually the config method need do nothing but assign constants into the passed parameter struct.)

If the indexed column is of a collatable data type, the index collation will be passed to all the support methods, using the standard PG_GET_COLLATION() mechanism.

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