66.1. Basic API Structure for Indexes
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## 66.1. Basic API Structure for Indexes

Each index access method is described by a row in the `pg_am` system catalog. The `pg_am` entry specifies a name and a handler function for the index access method. These entries can be created and deleted using the CREATE ACCESS METHOD and DROP ACCESS METHOD SQL commands.

An index access method handler function must be declared to accept a single argument of type `internal` and to return the pseudo-type `index_am_handler`. The argument is a dummy value that simply serves to prevent handler functions from being called directly from SQL commands. The result of the function must be a palloc'd struct of type `IndexAmRoutine`, which contains everything that the core code needs to know to make use of the index access method. The `IndexAmRoutine` struct, also called the access method's API struct, includes fields specifying assorted fixed properties of the access method, such as whether it can support multicolumn indexes. More importantly, it contains pointers to support functions for the access method, which do all of the real work to access indexes. These support functions are plain C functions and are not visible or callable at the SQL level. The support functions are described in Section 66.2.

The structure `IndexAmRoutine` is defined thus:

    typedef struct IndexAmRoutine
    {
        NodeTag     type;

        /*
         * Total number of strategies (operators) by which we can traverse/search
         * this AM.  Zero if AM does not have a fixed set of strategy assignments.
         */
        uint16      amstrategies;
        /* total number of support functions that this AM uses */
        uint16      amsupport;
        /* opclass options support function number or 0 */
        uint16      amoptsprocnum;
        /* does AM support ORDER BY indexed column's value? */
        bool        amcanorder;
        /* does AM support ORDER BY result of an operator on indexed column? */
        bool        amcanorderbyop;
        /* does AM support backward scanning? */
        bool        amcanbackward;
        /* does AM support UNIQUE indexes? */
        bool        amcanunique;
        /* does AM support multi-column indexes? */
        bool        amcanmulticol;
        /* does AM require scans to have a constraint on the first index column? */
        bool        amoptionalkey;
        /* does AM handle ScalarArrayOpExpr quals? */
        bool        amsearcharray;
        /* does AM handle IS NULL/IS NOT NULL quals? */
        bool        amsearchnulls;
        /* can index storage data type differ from column data type? */
        bool        amstorage;
        /* can an index of this type be clustered on? */
        bool        amclusterable;
        /* does AM handle predicate locks? */
        bool        ampredlocks;
        /* does AM support parallel scan? */
        bool        amcanparallel;
        /* does AM support columns included with clause INCLUDE? */
        bool        amcaninclude;
        /* does AM use maintenance_work_mem? */
        bool        amusemaintenanceworkmem;
        /* OR of parallel vacuum flags */
        uint8       amparallelvacuumoptions;
        /* type of data stored in index, or InvalidOid if variable */
        Oid         amkeytype;

        /* interface functions */
        ambuild_function ambuild;
        ambuildempty_function ambuildempty;
        aminsert_function aminsert;
        ambulkdelete_function ambulkdelete;
        amvacuumcleanup_function amvacuumcleanup;
        amcanreturn_function amcanreturn;   /* can be NULL */
        amcostestimate_function amcostestimate;
        amoptions_function amoptions;
        amproperty_function amproperty;     /* can be NULL */
        ambuildphasename_function ambuildphasename;   /* can be NULL */
        amvalidate_function amvalidate;
        amadjustmembers_function amadjustmembers; /* can be NULL */
        ambeginscan_function ambeginscan;
        amrescan_function amrescan;
        amgettuple_function amgettuple;     /* can be NULL */
        amgetbitmap_function amgetbitmap;   /* can be NULL */
        amendscan_function amendscan;
        ammarkpos_function ammarkpos;       /* can be NULL */
        amrestrpos_function amrestrpos;     /* can be NULL */

        /* interface functions to support parallel index scans */
        amestimateparallelscan_function amestimateparallelscan;    /* can be NULL */
        aminitparallelscan_function aminitparallelscan;    /* can be NULL */
        amparallelrescan_function amparallelrescan;    /* can be NULL */
    } IndexAmRoutine;

To be useful, an index access method must also have one or more operator families and operator classes defined in `pg_opfamily`, `pg_opclass`, `pg_amop`, and `pg_amproc`. These entries allow the planner to determine what kinds of query qualifications can be used with indexes of this access method. Operator families and classes are described in Section 40.16, which is prerequisite material for reading this chapter.

An individual index is defined by a `pg_class` entry that describes it as a physical relation, plus a `pg_index` entry that shows the logical content of the index — that is, the set of index columns it has and the semantics of those columns, as captured by the associated operator classes. The index columns (key values) can be either simple columns of the underlying table or expressions over the table rows. The index access method normally has no interest in where the index key values come from (it is always handed precomputed key values) but it will be very interested in the operator class information in `pg_index`. Both of these catalog entries can be accessed as part of the `Relation` data structure that is passed to all operations on the index.

Some of the flag fields of `IndexAmRoutine` have nonobvious implications. The requirements of `amcanunique` are discussed in Section 66.5. The `amcanmulticol` flag asserts that the access method supports multi-key-column indexes, while `amoptionalkey` asserts that it allows scans where no indexable restriction clause is given for the first index column. When `amcanmulticol` is false, `amoptionalkey` essentially says whether the access method supports full-index scans without any restriction clause. Access methods that support multiple index columns must support scans that omit restrictions on any or all of the columns after the first; however they are permitted to require some restriction to appear for the first index column, and this is signaled by setting `amoptionalkey` false. One reason that an index AM might set `amoptionalkey` false is if it doesn't index null values. Since most indexable operators are strict and hence cannot return true for null inputs, it is at first sight attractive to not store index entries for null values: they could never be returned by an index scan anyway. However, this argument fails when an index scan has no restriction clause for a given index column. In practice this means that indexes that have `amoptionalkey` true must index nulls, since the planner might decide to use such an index with no scan keys at all. A related restriction is that an index access method that supports multiple index columns must support indexing null values in columns after the first, because the planner will assume the index can be used for queries that do not restrict these columns. For example, consider an index on (a,b) and a query with `WHERE a = 4`. The system will assume the index can be used to scan for rows with `a = 4`, which is wrong if the index omits rows where `b` is null. It is, however, OK to omit rows where the first indexed column is null. An index access method that does index nulls may also set `amsearchnulls`, indicating that it supports `IS NULL` and `IS NOT NULL` clauses as search conditions.

The `amcaninclude` flag indicates whether the access method supports “included” columns, that is it can store (without processing) additional columns beyond the key column(s). The requirements of the preceding paragraph apply only to the key columns. In particular, the combination of `amcanmulticol`=`false` and `amcaninclude`=`true` is sensible: it means that there can only be one key column, but there can also be included column(s). Also, included columns must be allowed to be null, independently of `amoptionalkey`.

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