| PostgreSQL 9.1.14 Documentation | ||||
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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.
  
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 tables; 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 pg_am.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 52.5 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(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 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.
  
void
amcostestimate (PlannerInfo *root,
                IndexOptInfo *index,
                List *indexQuals,
                List *indexOrderBys,
                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 52.6, 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.
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 52.3, 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 indexscan, 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.
  
   The amgettuple function need only be provided if the access
   method supports "plain" index scans.  If it doesn't, the
   amgettuple field in its pg_am row must
   be set to zero.
  
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 52.3.
  
   The amgetbitmap function need only be provided if the access
   method supports "bitmap" index scans.  If it doesn't, the
   amgetbitmap field in its pg_am row must
   be set to zero.
  
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.
void 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.