The index construction and maintenance functions that an index access method must provide in IndexAmRoutine
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.
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);
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 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 59.5 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 checkUnique
is 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.
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 boolamvacuumcleanup
.
Because of limited maintenance_work_mem
, 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 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.
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);
Estimate the costs of an index scan. This function is described fully in Section 59.6, 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; most of the standard access methods 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.
bool amproperty (Oid index_oid, int attno, IndexAMProperty prop, const char *propname, bool *res, bool *isnull);
The 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 index_oid
and 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 pg_index_column_has_property
calls. 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 prop
is 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 *isnull
to 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 ereport
messages.
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 59.3, 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 nkeys
and 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 amrescan
.
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 keys
and/or 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 amgettuple
callers.
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 original indexed data for the index entry, in the form of an IndexTuple
pointer stored at scan->xs_itup
, with tuple descriptor scan->xs_itupdesc
. (Management of the data referenced by the pointer is the access method's responsibility. The data must remain good at least until the next amgettuple
, amrescan
, or amendscan
call for the scan.)
The 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. amgetbitmap
and amgettuple
cannot be used in the same index scan; there are other restrictions too when using amgetbitmap
, as explained in Section 59.3.
The 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.
void ammarkpos (IndexScanDesc scan);
Mark current scan position. The access method need only support one remembered scan position per scan.
The 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.
The 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.
Please login to continue.