Refactor GIN's handling of duplicate search entries.

The original coding could combine duplicate entries only when they
originated from the same qual condition.  In particular it could not
combine cases where multiple qual conditions all give rise to full-index
scan requests, which is an expensive case well worth optimizing.  Refactor
so that duplicates are recognized across all the quals.

src/include/access/gin_private.h

@@ -549,12 +549,19 @@ extern void ginPrepareDataScan(GinBtree btree, Relation index);
 /* ginscan.c */
 
 /*
- * GinScanKeyData describes a single GIN index qualification condition.
- * It contains one GinScanEntryData for each key datum extracted from
- * the qual by the extractQueryFn or added implicitly by ginFillScanKey.
- * nentries is the true number of entries, nuserentries is the number
- * that extractQueryFn returned (which is what we report to consistentFn).
- * The "user" entries must come first.
+ * GinScanKeyData describes a single GIN index qualifier expression.
+ *
+ * From each qual expression, we extract one or more specific index search
+ * conditions, which are represented by GinScanEntryData.  It's quite
+ * possible for identical search conditions to be requested by more than
+ * one qual expression, in which case we merge such conditions to have just
+ * one unique GinScanEntry --- this is particularly important for efficiency
+ * when dealing with full-index-scan entries.  So there can be multiple
+ * GinScanKeyData.scanEntry pointers to the same GinScanEntryData.
+ *
+ * In each GinScanKeyData, nentries is the true number of entries, while
+ * nuserentries is the number that extractQueryFn returned (which is what
+ * we report to consistentFn).  The "user" entries must come first.
  */
 typedef struct GinScanKeyData *GinScanKey;
 
@@ -567,10 +574,10 @@ typedef struct GinScanKeyData
 	/* Number of entries that extractQueryFn and consistentFn know about */
 	uint32		nuserentries;
 
-	/* array of GinScanEntryData, one per key datum */
-	GinScanEntry scanEntry;
+	/* array of GinScanEntry pointers, one per extracted search condition */
+	GinScanEntry *scanEntry;
 
-	/* array of ItemPointer result, reported to consistentFn */
+	/* array of check flags, reported to consistentFn */
 	bool	   *entryRes;
 
 	/* other data needed for calling consistentFn */
@@ -583,22 +590,21 @@ typedef struct GinScanKeyData
 	int32		searchMode;
 	OffsetNumber attnum;
 
-	/* scan status data */
+	/*
+	 * Match status data.  curItem is the TID most recently tested (could be
+	 * a lossy-page pointer).  curItemMatches is TRUE if it passes the
+	 * consistentFn test; if so, recheckCurItem is the recheck flag.
+	 * isFinished means that all the input entry streams are finished, so
+	 * this key cannot succeed for any later TIDs.
+	 */
 	ItemPointerData curItem;
+	bool		curItemMatches;
 	bool		recheckCurItem;
-
-	bool		firstCall;
 	bool		isFinished;
 } GinScanKeyData;
 
 typedef struct GinScanEntryData
 {
-	/* link to any preceding identical entry in current scan key */
-	GinScanEntry master;
-
-	/* ptr to value reported to consistentFn, points to parent->entryRes[i] */
-	bool	   *pval;
-
 	/* query key and other information from extractQueryFn */
 	Datum		queryKey;
 	GinNullCategory queryCategory;
@@ -634,10 +640,14 @@ typedef struct GinScanOpaqueData
 	MemoryContext tempCtx;
 	GinState	ginstate;
 
-	GinScanKey	keys;
+	GinScanKey	keys;			/* one per scan qualifier expr */
 	uint32		nkeys;
-	bool		isVoidRes;		/* true if ginstate.extractQueryFn guarantees
-								 * that nothing will be found */
+
+	GinScanEntry *entries;		/* one per index search condition */
+	uint32		totalentries;
+	uint32		allocentries;	/* allocated length of entries[] */
+
+	bool		isVoidRes;		/* true if query is unsatisfiable */
 } GinScanOpaqueData;
 
 typedef GinScanOpaqueData *GinScanOpaque;