Teach SP-GiST to do index-only scans.

Operator classes can specify whether or not they support this; this
preserves the flexibility to use lossy representations within an index.

In passing, move constant data about a given index into the rd_amcache
cache area, instead of doing fresh lookups each time we start an index
operation.  This is mainly to try to make sure that spgcanreturn() has
insignificant cost; I still don't have any proof that it matters for
actual index accesses.  Also, get rid of useless copying of FmgrInfo
pointers; we can perfectly well use the relcache's versions in-place.

2 files changed, 26 insertions, 9 deletions

diff --git a/src/include/access/spgist.h b/src/include/access/spgist.h
index df6fec6cf45..d2f0a72c36b 100644
--- a/src/include/access/spgist.h
+++ b/src/include/access/spgist.h
@@ -43,6 +43,7 @@ typedef struct spgConfigOut
 {
 	Oid			prefixType;		/* Data type of inner-tuple prefixes */
 	Oid			labelType;		/* Data type of inner-tuple node labels */
+	bool		canReturnData;	/* Opclass can reconstruct original data */
 	bool		longValuesOK;	/* Opclass can cope with values > 1 page */
 } spgConfigOut;
 
@@ -132,6 +133,7 @@ typedef struct spgInnerConsistentIn
 
 	Datum		reconstructedValue;		/* value reconstructed at parent */
 	int			level;			/* current level (counting from zero) */
+	bool		returnData;		/* original data must be returned? */
 
 	/* Data from current inner tuple */
 	bool		allTheSame;		/* tuple is marked all-the-same? */
@@ -159,12 +161,14 @@ typedef struct spgLeafConsistentIn
 
 	Datum		reconstructedValue;		/* value reconstructed at parent */
 	int			level;			/* current level (counting from zero) */
+	bool		returnData;		/* original data must be returned? */
 
 	Datum		leafDatum;		/* datum in leaf tuple */
 } spgLeafConsistentIn;
 
 typedef struct spgLeafConsistentOut
 {
+	Datum		leafValue;		/* reconstructed original data, if any */
 	bool		recheck;		/* set true if operator must be rechecked */
 } spgLeafConsistentOut;
 
diff --git a/src/include/access/spgist_private.h b/src/include/access/spgist_private.h
index 5c57799f09c..ec6d2d07fc7 100644
--- a/src/include/access/spgist_private.h
+++ b/src/include/access/spgist_private.h
@@ -71,11 +71,11 @@ typedef struct SpGistLastUsedPage
 	int			freeSpace;		/* its free space (could be obsolete!) */
 } SpGistLastUsedPage;
 
-typedef struct SpGistCache
+typedef struct SpGistLUPCache
 {
 	SpGistLastUsedPage innerPage[3];	/* one per triple-parity group */
 	SpGistLastUsedPage leafPage;
-} SpGistCache;
+} SpGistLUPCache;
 
 /*
  * metapage
@@ -83,7 +83,7 @@ typedef struct SpGistCache
 typedef struct SpGistMetaPageData
 {
 	uint32		magicNumber;	/* for identity cross-check */
-	SpGistCache lastUsedPages;	/* shared storage of last-used info */
+	SpGistLUPCache lastUsedPages;	/* shared storage of last-used info */
 } SpGistMetaPageData;
 
 #define SPGIST_MAGIC_NUMBER (0xBA0BABED)
@@ -112,12 +112,6 @@ typedef struct SpGistState
 	SpGistTypeDesc attPrefixType;	/* type of inner-tuple prefix values */
 	SpGistTypeDesc attLabelType;	/* type of node label values */
 
-	/* lookup data for the opclass support functions, except config */
-	FmgrInfo	chooseFn;
-	FmgrInfo	picksplitFn;
-	FmgrInfo	innerConsistentFn;
-	FmgrInfo	leafConsistentFn;
-
 	char	   *deadTupleStorage;	/* workspace for spgFormDeadTuple */
 
 	TransactionId myXid;		/* XID to use when creating a redirect tuple */
@@ -144,10 +138,13 @@ typedef struct SpGistScanOpaqueData
 	int64		ntids;			/* number of TIDs passed to bitmap */
 
 	/* These fields are only used in amgettuple scans: */
+	bool		want_itup;		/* are we reconstructing tuples? */
+	TupleDesc	indexTupDesc;	/* if so, tuple descriptor for them */
 	int			nPtrs;			/* number of TIDs found on current page */
 	int			iPtr;			/* index for scanning through same */
 	ItemPointerData heapPtrs[MaxIndexTuplesPerPage]; /* TIDs from cur page */
 	bool		recheck[MaxIndexTuplesPerPage];		/* their recheck flags */
+	IndexTuple	indexTups[MaxIndexTuplesPerPage];	/* reconstructed tuples */
 
 	/*
 	 * Note: using MaxIndexTuplesPerPage above is a bit hokey since
@@ -158,6 +155,21 @@ typedef struct SpGistScanOpaqueData
 
 typedef SpGistScanOpaqueData *SpGistScanOpaque;
 
+/*
+ * This struct is what we actually keep in index->rd_amcache.  It includes
+ * static configuration information as well as the lastUsedPages cache.
+ */
+typedef struct SpGistCache
+{
+	spgConfigOut config;		/* filled in by opclass config method */
+
+	SpGistTypeDesc attType;			/* type of input data and leaf values */
+	SpGistTypeDesc attPrefixType;	/* type of inner-tuple prefix values */
+	SpGistTypeDesc attLabelType;	/* type of node label values */
+
+	SpGistLUPCache lastUsedPages;	/* local storage of last-used info */
+} SpGistCache;
+
 
 /*
  * SPGiST tuple types.  Note: inner, leaf, and dead tuple structs
@@ -570,6 +582,7 @@ typedef struct spgxlogVacuumRedirect
 #define GBUF_INNER_PARITY(x)	((x) % 3)
 
 /* spgutils.c */
+extern SpGistCache *spgGetCache(Relation index);
 extern void initSpGistState(SpGistState *state, Relation index);
 extern Buffer SpGistNewBuffer(Relation index);
 extern void SpGistUpdateMetaPage(Relation index);