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In an index scan, the index access method is responsible for regurgitating the TIDs of all the tuples it has been told about that match thescan keys. The access method isnot involved in actually fetching those tuples from the index's parent table, nor in determining whether they pass the scan's visibility test or other conditions.

A scan key is the internal representation of aWHERE clause of the formindex_keyoperatorconstant, where the index key is one of the columns of the index and the operator is one of the members of the operator family associated with that index column. An index scan has zero or more scan keys, which are implicitly ANDed — the returned tuples are expected to satisfy all the indicated conditions.

The access method can report that the index islossy, or requires rechecks, for a particular query. This implies that the index scan will return all the entries that pass the scan key, plus possibly additional entries that do not. The core system's index-scan machinery will then apply the index conditions again to the heap tuple to verify whether or not it really should be selected. If the recheck option is not specified, the index scan must return exactly the set of matching entries.

Note that it is entirely up to the access method to ensure that it correctly finds all and only the entries passing all the given scan keys. Also, the core system will simply hand off all theWHERE clauses that match the index keys and operator families, without any semantic analysis to determine whether they are redundant or contradictory. As an example, givenWHERE x > 4 AND x > 14 wherex is a b-tree indexed column, it is left to the b-treeamrescan function to realize that the first scan key is redundant and can be discarded. The extent of preprocessing needed duringamrescan will depend on the extent to which the index access method needs to reduce the scan keys to a“normalized” form.

Some access methods return index entries in a well-defined order, others do not. There are actually two different ways that an access method can support sorted output:

Theamgettuple function has adirection argument, which can be eitherForwardScanDirection (the normal case) orBackwardScanDirection. If the first call afteramrescan specifiesBackwardScanDirection, then the set of matching index entries is to be scanned back-to-front rather than in the normal front-to-back direction, soamgettuple must return the last matching tuple in the index, rather than the first one as it normally would. (This will only occur for access methods that setamcanorder to true.) After the first call,amgettuple must be prepared to advance the scan in either direction from the most recently returned entry. (But ifamcanbackward is false, all subsequent calls will have the same direction as the first one.)

Access methods that support ordered scans must support“marking” a position in a scan and later returning to the marked position. The same position might be restored multiple times. However, only one position need be remembered per scan; a newammarkpos call overrides the previously marked position. An access method that does not support ordered scans need not provideammarkpos andamrestrpos functions inIndexAmRoutine; set those pointers to NULL instead.

Both the scan position and the mark position (if any) must be maintained consistently in the face of concurrent insertions or deletions in the index. It is OK if a freshly-inserted entry is not returned by a scan that would have found the entry if it had existed when the scan started, or for the scan to return such an entry upon rescanning or backing up even though it had not been returned the first time through. Similarly, a concurrent delete might or might not be reflected in the results of a scan. What is important is that insertions or deletions not cause the scan to miss or multiply return entries that were not themselves being inserted or deleted.

If the index stores the original indexed data values (and not some lossy representation of them), it is useful to supportindex-only scans, in which the index returns the actual data not just the TID of the heap tuple. This will only avoid I/O if the visibility map shows that the TID is on an all-visible page; else the heap tuple must be visited anyway to check MVCC visibility. But that is no concern of the access method's.

Instead of usingamgettuple, an index scan can be done withamgetbitmap to fetch all tuples in one call. This can be noticeably more efficient thanamgettuple because it allows avoiding lock/unlock cycles within the access method. In principleamgetbitmap should have the same effects as repeatedamgettuple calls, but we impose several restrictions to simplify matters. First of all,amgetbitmap returns all tuples at once and marking or restoring scan positions isn't supported. Secondly, the tuples are returned in a bitmap which doesn't have any specific ordering, which is whyamgetbitmap doesn't take adirection argument. (Ordering operators will never be supplied for such a scan, either.) Also, there is no provision for index-only scans withamgetbitmap, since there is no way to return the contents of index tuples. Finally,amgetbitmap does not guarantee any locking of the returned tuples, with implications spelled out inSection 62.4.

Note that it is permitted for an access method to implement onlyamgetbitmap and notamgettuple, or vice versa, if its internal implementation is unsuited to one API or the other.