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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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2
src/include/nodes/execnodes.h
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@ -1226,6 +1226,7 @@ typedef struct
* RuntimeContext expr context for evaling runtime Skeys
* RelationDesc index relation descriptor
* ScanDesc index scan descriptor
* VMBuffer buffer in use for visibility map testing, if any
* ----------------
*/
typedef struct IndexScanState
@ -1242,6 +1243,7 @@ typedef struct IndexScanState
ExprContext *iss_RuntimeContext;
Relation iss_RelationDesc;
IndexScanDesc iss_ScanDesc;
Buffer iss_VMBuffer;
} IndexScanState;
/* ----------------