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Support index-only scans using the visibility map to avoid heap fetches.

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When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.
This commit is contained in:
Tom Lane
2011-10-07 20:13:02 -04:00
parent caa1054df8
commit a2822fb933
34 changed files with 704 additions and 203 deletions
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3
src/include/access/genam.h
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@@ -144,6 +144,9 @@ extern void index_rescan(IndexScanDesc scan,
extern void index_endscan(IndexScanDesc scan);
extern void index_markpos(IndexScanDesc scan);
extern void index_restrpos(IndexScanDesc scan);
extern ItemPointer index_getnext_tid(IndexScanDesc scan,
ScanDirection direction);
extern HeapTuple index_fetch_heap(IndexScanDesc scan);
extern HeapTuple index_getnext(IndexScanDesc scan, ScanDirection direction);
extern int64 index_getbitmap(IndexScanDesc scan, TIDBitmap *bitmap);

5
src/include/access/relscan.h
View File

@@ -16,6 +16,7 @@
#include "access/genam.h"
#include "access/heapam.h"
#include "access/itup.h"
typedef struct HeapScanDescData
@@ -66,6 +67,7 @@ typedef struct IndexScanDescData
int numberOfOrderBys; /* number of ordering operators */
ScanKey keyData; /* array of index qualifier descriptors */
ScanKey orderByData; /* array of ordering op descriptors */
bool xs_want_itup; /* caller requests index tuples */
/* signaling to index AM about killing index tuples */
bool kill_prior_tuple; /* last-returned tuple is dead */
@@ -76,6 +78,9 @@ typedef struct IndexScanDescData
/* index access method's private state */
void *opaque; /* access-method-specific info */
/* in an index-only scan, this is valid after a successful amgettuple */
IndexTuple xs_itup; /* index tuple returned by AM, or NULL */
/* xs_ctup/xs_cbuf/xs_recheck are valid after a successful index_getnext */
HeapTupleData xs_ctup; /* current heap tuple, if any */
Buffer xs_cbuf; /* current heap buffer in scan, if any */