49.2. Index Access Method Functions

The index construction and maintenance functions that an index access method must provide are:

IndexBuildResult *
ambuild (Relation heapRelation,
         Relation indexRelation,
         IndexInfo *indexInfo);

Build a new index. The index relation has been physically created, but is empty. It must be filled in with whatever fixed data the access method requires, plus entries for all tuples already existing in the table. Ordinarily the ambuild function will call IndexBuildHeapScan() to scan the table for existing tuples and compute the keys that need to be inserted into the index. The function must return a palloc'd struct containing statistics about the new index.

aminsert (Relation indexRelation,
          Datum *values,
          bool *isnull,
          ItemPointer heap_tid,
          Relation heapRelation,
          bool check_uniqueness);

Insert a new tuple into an existing index. The values and isnull arrays give the key values to be indexed, and heap_tid is the TID to be indexed. If the access method supports unique indexes (its pg_am.amcanunique flag is true) then check_uniqueness may be true, in which case the access method must verify that there is no conflicting row; this is the only situation in which the access method normally needs the heapRelation parameter. See Section 49.5 for details. The result is TRUE if an index entry was inserted, FALSE if not. (A FALSE result does not denote an error condition, but is used for cases such as an index AM refusing to index a NULL.)

IndexBulkDeleteResult *
ambulkdelete (IndexVacuumInfo *info,
              IndexBulkDeleteResult *stats,
              IndexBulkDeleteCallback callback,
              void *callback_state);

Delete tuple(s) from the index. This is a "bulk delete" operation that is intended to be implemented by scanning the whole index and checking each entry to see if it should be deleted. The passed-in callback function must be called, in the style callback(TID, callback_state) returns bool, to determine whether any particular index entry, as identified by its referenced TID, is to be deleted. Must return either NULL or a palloc'd struct containing statistics about the effects of the deletion operation. It is OK to return NULL if no information needs to be passed on to amvacuumcleanup.

Because of limited maintenance_work_mem, ambulkdelete may need to be called more than once when many tuples are to be deleted. The stats argument is the result of the previous call for this index (it is NULL for the first call within a VACUUM operation). This allows the AM to accumulate statistics across the whole operation. Typically, ambulkdelete will modify and return the same struct if the passed stats is not null.

IndexBulkDeleteResult *
amvacuumcleanup (IndexVacuumInfo *info,
                 IndexBulkDeleteResult *stats);

Clean up after a VACUUM operation (zero or more ambulkdelete calls). This does not have to do anything beyond returning index statistics, but it may perform bulk cleanup such as reclaiming empty index pages. stats is whatever the last ambulkdelete call returned, or NULL if ambulkdelete was not called because no tuples needed to be deleted. If the result is not NULL it must be a palloc'd struct. The statistics it contains will be used to update pg_class, and will be reported by VACUUM if VERBOSE is given. It is OK to return NULL if the index was not changed at all during the VACUUM operation, but otherwise correct stats should be returned.

amcostestimate (PlannerInfo *root,
                IndexOptInfo *index,
                List *indexQuals,
                RelOptInfo *outer_rel,
                Cost *indexStartupCost,
                Cost *indexTotalCost,
                Selectivity *indexSelectivity,
                double *indexCorrelation);

Estimate the costs of an index scan. This function is described fully in Section 49.6, below.

bytea *
amoptions (ArrayType *reloptions,
           bool validate);

Parse and validate the reloptions array for an index. This is called only when a non-null reloptions array exists for the index. reloptions is a text array containing entries of the form name=value. The function should construct a bytea value, which will be copied into the rd_options field of the index's relcache entry. The data contents of the bytea value are open for the access method to define, but the standard access methods currently all use struct StdRdOptions. When validate is true, the function should report a suitable error message if any of the options are unrecognized or have invalid values; when validate is false, invalid entries should be silently ignored. (validate is false when loading options already stored in pg_catalog; an invalid entry could only be found if the access method has changed its rules for options, and in that case ignoring obsolete entries is appropriate.) It is OK to return NULL if default behavior is wanted.

The purpose of an index, of course, is to support scans for tuples matching an indexable WHERE condition, often called a qualifier or scan key. The semantics of index scanning are described more fully in Section 49.3, below. The scan-related functions that an index access method must provide are:

ambeginscan (Relation indexRelation,
             int nkeys,
             ScanKey key);

Begin a new scan. The key array (of length nkeys) describes the scan key(s) for the index scan. The result must be a palloc'd struct. For implementation reasons the index access method must create this struct by calling RelationGetIndexScan(). In most cases ambeginscan itself does little beyond making that call; the interesting parts of index-scan startup are in amrescan.

amgettuple (IndexScanDesc scan,
            ScanDirection direction);

Fetch the next tuple in the given scan, moving in the given direction (forward or backward in the index). Returns TRUE if a tuple was obtained, FALSE if no matching tuples remain. In the TRUE case the tuple TID is stored into the scan structure. Note that "success" means only that the index contains an entry that matches the scan keys, not that the tuple necessarily still exists in the heap or will pass the caller's snapshot test.

amgetmulti (IndexScanDesc scan,
            ItemPointer tids,
            int32 max_tids,
            int32 *returned_tids);

Fetch multiple tuples in the given scan. Returns TRUE if the scan should continue, FALSE if no matching tuples remain. tids points to a caller-supplied array of max_tids ItemPointerData records, which the call fills with TIDs of matching tuples. *returned_tids is set to the number of TIDs actually returned. This can be less than max_tids, or even zero, even when the return value is TRUE. (This provision allows the access method to choose the most efficient stopping points in its scan, for example index page boundaries.) amgetmulti and amgettuple cannot be used in the same index scan; there are other restrictions too when using amgetmulti, as explained in Section 49.3.

amrescan (IndexScanDesc scan,
          ScanKey key);

Restart the given scan, possibly with new scan keys (to continue using the old keys, NULL is passed for key). Note that it is not possible for the number of keys to be changed. In practice the restart feature is used when a new outer tuple is selected by a nested-loop join and so a new key comparison value is needed, but the scan key structure remains the same. This function is also called by RelationGetIndexScan(), so it is used for initial setup of an index scan as well as rescanning.

amendscan (IndexScanDesc scan);

End a scan and release resources. The scan struct itself should not be freed, but any locks or pins taken internally by the access method must be released.

ammarkpos (IndexScanDesc scan);

Mark current scan position. The access method need only support one remembered scan position per scan.

amrestrpos (IndexScanDesc scan);

Restore the scan to the most recently marked position.

By convention, the pg_proc entry for an index access method function should show the correct number of arguments, but declare them all as type internal (since most of the arguments have types that are not known to SQL, and we don't want users calling the functions directly anyway). The return type is declared as void, internal, or boolean as appropriate. The only exception is amoptions, which should be correctly declared as taking text[] and bool and returning bytea. This provision allows client code to execute amoptions to test validity of options settings.