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.

Author: tglsfdc

doc/src/sgml/spgist.sgml

@@ -145,6 +145,7 @@ 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;
 </programlisting>
@@ -159,6 +160,8 @@ typedef struct spgConfigOut
       <structfield>prefixType</> can be set to <literal>VOIDOID</>.
       Likewise, for operator classes that do not use node labels,
       <structfield>labelType</> can be set to <literal>VOIDOID</>.
+      <structfield>canReturnData</> should be set true if the operator class
+      is capable of reconstructing the originally-supplied index value.
       <structfield>longValuesOK</> should be set true only when the
       <structfield>attType</> is of variable length and the operator
       class is capable of segmenting long values by repeated suffixing
@@ -441,6 +444,7 @@ 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? */
@@ -467,6 +471,9 @@ typedef struct spgInnerConsistentOut
        parent level.
        <structfield>level</> is the current inner tuple's level, starting at
        zero for the root level.
+       <structfield>returnData</> is <literal>true</> if reconstructed data is
+       required for this query; this will only be so if the
+       <function>config</> function asserted <structfield>canReturnData</>.
        <structfield>allTheSame</> is true if the current inner tuple is
        marked <quote>all-the-same</>; in this case all the nodes have the
        same label (if any) and so either all or none of them match the query
@@ -525,12 +532,14 @@ 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;
 </programlisting>
@@ -543,13 +552,19 @@ typedef struct spgLeafConsistentOut
        parent level.
        <structfield>level</> is the current leaf tuple's level, starting at
        zero for the root level.
+       <structfield>returnData</> is <literal>true</> if reconstructed data is
+       required for this query; this will only be so if the
+       <function>config</> function asserted <structfield>canReturnData</>.
        <structfield>leafDatum</> is the key value stored in the current
        leaf tuple.
       </para>
 
       <para>
        The function must return <literal>true</> if the leaf tuple matches the
        query, or <literal>false</> if not.  In the <literal>true</> case,
+       if <structfield>returnData</> is <literal>true</> then
+       <structfield>leafValue</> must be set to the value originally supplied
+       to be indexed for this leaf tuple.  Also,
        <structfield>recheck</> may be set to <literal>true</> if the match
        is uncertain and so the operator must be re-applied to the actual heap
        tuple to verify the match.

src/backend/access/spgist/spgdoinsert.c

@@ -15,6 +15,7 @@
 
 #include "postgres.h"
 
+#include "access/genam.h"
 #include "access/spgist_private.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
@@ -678,6 +679,7 @@ doPickSplit(Relation index, SpGistState *state,
 	bool		insertedNew = false;
 	spgPickSplitIn in;
 	spgPickSplitOut out;
+	FmgrInfo   *procinfo;
 	bool		includeNew;
 	int			i,
 				max,
@@ -816,7 +818,8 @@ doPickSplit(Relation index, SpGistState *state,
 	 */
 	memset(&out, 0, sizeof(out));
 
-	FunctionCall2Coll(&state->picksplitFn,
+	procinfo = index_getprocinfo(index, 1, SPGIST_PICKSPLIT_PROC);
+	FunctionCall2Coll(procinfo,
 					  index->rd_indcollation[0],
 					  PointerGetDatum(&in),
 					  PointerGetDatum(&out));
@@ -1944,6 +1947,7 @@ spgdoinsert(Relation index, SpGistState *state,
 			SpGistInnerTuple innerTuple;
 			spgChooseIn in;
 			spgChooseOut out;
+			FmgrInfo   *procinfo;
 
 			/*
 			 * spgAddNode and spgSplitTuple cases will loop back to here to
@@ -1968,7 +1972,8 @@ spgdoinsert(Relation index, SpGistState *state,
 
 			memset(&out, 0, sizeof(out));
 
-			FunctionCall2Coll(&state->chooseFn,
+			procinfo = index_getprocinfo(index, 1, SPGIST_CHOOSE_PROC);
+			FunctionCall2Coll(procinfo,
 							  index->rd_indcollation[0],
 							  PointerGetDatum(&in),
 							  PointerGetDatum(&out));

src/backend/access/spgist/spgkdtreeproc.c

@@ -30,6 +30,7 @@ spg_kd_config(PG_FUNCTION_ARGS)
 
 	cfg->prefixType = FLOAT8OID;
 	cfg->labelType = VOIDOID;	/* we don't need node labels */
+	cfg->canReturnData = true;
 	cfg->longValuesOK = false;
 	PG_RETURN_VOID();
 }

src/backend/access/spgist/spgquadtreeproc.c

@@ -30,6 +30,7 @@ spg_quad_config(PG_FUNCTION_ARGS)
 
 	cfg->prefixType = POINTOID;
 	cfg->labelType = VOIDOID;	/* we don't need node labels */
+	cfg->canReturnData = true;
 	cfg->longValuesOK = false;
 	PG_RETURN_VOID();
 }
@@ -324,6 +325,9 @@ spg_quad_leaf_consistent(PG_FUNCTION_ARGS)
 	/* all tests are exact */
 	out->recheck = false;
 
+	/* leafDatum is what it is... */
+	out->leafValue = in->leafDatum;
+
 	switch (in->strategy)
 	{
 		case RTLeftStrategyNumber: