58.2. Index Access Method Functions
The index construction and maintenance functions that an index access method must provide in
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.
void ambuildempty (Relation indexRelation);
Build an empty index, and write it to the initialization fork (
INIT_FORKNUM) of the given relation. This method is called only for unlogged indexes; the empty index written to the initialization fork will be copied over the main relation fork on each server restart.
bool aminsert (Relation indexRelation, Datum *values, bool *isnull, ItemPointer heap_tid, Relation heapRelation, IndexUniqueCheck checkUnique, IndexInfo *indexInfo);
Insert a new tuple into an existing index. The
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
amcanunique flag is true) then
checkUnique indicates the type of uniqueness check to perform. This varies depending on whether the unique constraint is deferrable; see Section 58.5 for details. Normally the access method only needs the
heapRelation parameter when performing uniqueness checking (since then it will have to look into the heap to verify tuple liveness).
The function's Boolean result value is significant only when
UNIQUE_CHECK_PARTIAL. In this case a TRUE result means the new entry is known unique, whereas FALSE means it might be non-unique (and a deferred uniqueness check must be scheduled). For other cases a constant FALSE result is recommended.
Some indexes might not index all tuples. If the tuple is not to be indexed,
aminsert should just return without doing anything.
If the index AM wishes to cache data across successive index insertions within a SQL statement, it can allocate space in
indexInfo->ii_Context and store a pointer to the data in
indexInfo->ii_AmCache (which will be NULL initially).
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(, 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
TID, callback_state) returns bool
Because of limited
ambulkdelete might 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 might 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
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.
As of PostgreSQL 8.4,
amvacuumcleanup will also be called at completion of an
ANALYZE operation. In this case
stats is always NULL and any return value will be ignored. This case can be distinguished by checking
info->analyze_only. It is recommended that the access method do nothing except post-insert cleanup in such a call, and that only in an autovacuum worker process.
bool amcanreturn (Relation indexRelation, int attno);
Check whether the index can support index-only scans on the given column, by returning the indexed column values for an index entry in the form of an
IndexTuple. The attribute number is 1-based, i.e., the first column's attno is 1. Returns TRUE if supported, else FALSE. If the access method does not support index-only scans at all, the
amcanreturn field in its
IndexAmRoutine struct can be set to NULL.
void amcostestimate (PlannerInfo *root, IndexPath *path, double loop_count, Cost *indexStartupCost, Cost *indexTotalCost, Selectivity *indexSelectivity, double *indexCorrelation, double *indexPages);
Estimate the costs of an index scan. This function is described fully in Section 58.6, 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
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; most of the standard access methods use struct
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.
bool amproperty (Oid index_oid, int attno, IndexAMProperty prop, const char *propname, bool *res, bool *isnull);
amproperty method allows index access methods to override the default behavior of
pg_index_column_has_property and related functions. If the access method does not have any special behavior for index property inquiries, the
amproperty field in its
IndexAmRoutine struct can be set to NULL. Otherwise, the
amproperty method will be called with
attno both zero for
pg_indexam_has_property calls, or with
index_oid valid and
attno zero for
pg_index_has_property calls, or with
index_oid valid and
attno greater than zero for
prop is an enum value identifying the property being tested, while
propname is the original property name string. If the core code does not recognize the property name then
AMPROP_UNKNOWN. Access methods can define custom property names by checking
propname for a match (use
pg_strcasecmp to match, for consistency with the core code); for names known to the core code, it's better to inspect
prop. If the
amproperty method returns
true then it has determined the property test result: it must set
*res to the boolean value to return, or set
true to return a NULL. (Both of the referenced variables are initialized to
false before the call.) If the
amproperty method returns
false then the core code will proceed with its normal logic for determining the property test result.
Access methods that support ordering operators should implement
AMPROP_DISTANCE_ORDERABLE property testing, as the core code does not know how to do that and will return NULL. It may also be advantageous to implement
AMPROP_RETURNABLE testing, if that can be done more cheaply than by opening the index and calling
amcanreturn, which is the core code's default behavior. The default behavior should be satisfactory for all other standard properties.
bool amvalidate (Oid opclassoid);
Validate the catalog entries for the specified operator class, so far as the access method can reasonably do that. For example, this might include testing that all required support functions are provided. The
amvalidate function must return false if the opclass is invalid. Problems should be reported with
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 58.3, below. An index access method can support “plain” index scans, “bitmap” index scans, or both. The scan-related functions that an index access method must or may provide are:
IndexScanDesc ambeginscan (Relation indexRelation, int nkeys, int norderbys);
Prepare for an index scan. The
norderbys parameters indicate the number of quals and ordering operators that will be used in the scan; these may be useful for space allocation purposes. Note that the actual values of the scan keys aren't provided yet. 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 does little beyond making that call and perhaps acquiring locks; the interesting parts of index-scan startup are in
void amrescan (IndexScanDesc scan, ScanKey keys, int nkeys, ScanKey orderbys, int norderbys);
Start or restart an index scan, possibly with new scan keys. (To restart using previously-passed keys, NULL is passed for
orderbys.) Note that it is not allowed for the number of keys or order-by operators to be larger than what was passed to
ambeginscan. 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.
boolean 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. On success,
amgettuple must also set
scan->xs_recheck to TRUE or FALSE. FALSE means it is certain that the index entry matches the scan keys. TRUE means this is not certain, and the conditions represented by the scan keys must be rechecked against the heap tuple after fetching it. This provision supports “lossy” index operators. Note that rechecking will extend only to the scan conditions; a partial index predicate (if any) is never rechecked by
If the index supports index-only scans (i.e.,
amcanreturn returns TRUE for it), then on success the AM must also check
scan->xs_want_itup, and if that is true it must return the originally indexed data for the index entry. The data can be returned in the form of an
IndexTuple pointer stored at
scan->xs_itup, with tuple descriptor
scan->xs_itupdesc; or in the form of a
HeapTuple pointer stored at
scan->xs_hitup, with tuple descriptor
scan->xs_hitupdesc. (The latter format should be used when reconstructing data that might possibly not fit into an
IndexTuple.) In either case, management of the data referenced by the pointer is the access method's responsibility. The data must remain good at least until the next
amendscan call for the scan.
amgettuple function need only be provided if the access method supports “plain” index scans. If it doesn't, the
amgettuple field in its
IndexAmRoutine struct must be set to NULL.
int64 amgetbitmap (IndexScanDesc scan, TIDBitmap *tbm);
Fetch all tuples in the given scan and add them to the caller-supplied
TIDBitmap (that is, OR the set of tuple IDs into whatever set is already in the bitmap). The number of tuples fetched is returned (this might be just an approximate count, for instance some AMs do not detect duplicates). While inserting tuple IDs into the bitmap,
amgetbitmap can indicate that rechecking of the scan conditions is required for specific tuple IDs. This is analogous to the
xs_recheck output parameter of
amgettuple. Note: in the current implementation, support for this feature is conflated with support for lossy storage of the bitmap itself, and therefore callers recheck both the scan conditions and the partial index predicate (if any) for recheckable tuples. That might not always be true, however.
amgettuple cannot be used in the same index scan; there are other restrictions too when using
amgetbitmap, as explained in Section 58.3.
amgetbitmap function need only be provided if the access method supports “bitmap” index scans. If it doesn't, the
amgetbitmap field in its
IndexAmRoutine struct must be set to NULL.
void 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, as well as any other memory allocated by
ambeginscan and other scan-related functions.
void ammarkpos (IndexScanDesc scan);
Mark current scan position. The access method need only support one remembered scan position per scan.
ammarkpos function need only be provided if the access method supports ordered scans. If it doesn't, the
ammarkpos field in its
IndexAmRoutine struct may be set to NULL.
void amrestrpos (IndexScanDesc scan);
Restore the scan to the most recently marked position.
amrestrpos function need only be provided if the access method supports ordered scans. If it doesn't, the
amrestrpos field in its
IndexAmRoutine struct may be set to NULL.
In addition to supporting ordinary index scans, some types of index may wish to support parallel index scans, which allow multiple backends to cooperate in performing an index scan. The index access method should arrange things so that each cooperating process returns a subset of the tuples that would be performed by an ordinary, non-parallel index scan, but in such a way that the union of those subsets is equal to the set of tuples that would be returned by an ordinary, non-parallel index scan. Furthermore, while there need not be any global ordering of tuples returned by a parallel scan, the ordering of that subset of tuples returned within each cooperating backend must match the requested ordering. The following functions may be implemented to support parallel index scans:
Size amestimateparallelscan (void);
Estimate and return the number of bytes of dynamic shared memory which the access method will be needed to perform a parallel scan. (This number is in addition to, not in lieu of, the amount of space needed for AM-independent data in
It is not necessary to implement this function for access methods which do not support parallel scans or for which the number of additional bytes of storage required is zero.
void aminitparallelscan (void *target);
This function will be called to initialize dynamic shared memory at the beginning of a parallel scan.
target will point to at least the number of bytes previously returned by
amestimateparallelscan, and this function may use that amount of space to store whatever data it wishes.
It is not necessary to implement this function for access methods which do not support parallel scans or in cases where the shared memory space required needs no initialization.
void amparallelrescan (IndexScanDesc scan);
This function, if implemented, will be called when a parallel index scan must be restarted. It should reset any shared state set up by
aminitparallelscan such that the scan will be restarted from the beginning.