Extend index AM API for parallel index scans.

This patch doesn't actually make any index AM parallel-aware, but it
provides the necessary functions at the AM layer to do so.

Rahila Syed, Amit Kapila, Robert Haas

src/include/access/amapi.h

@@ -137,6 +137,18 @@ typedef void (*ammarkpos_function) (IndexScanDesc scan);
 /* restore marked scan position */
 typedef void (*amrestrpos_function) (IndexScanDesc scan);
 
+/*
+ * Callback function signatures - for parallel index scans.
+ */
+
+/* estimate size of parallel scan descriptor */
+typedef Size (*amestimateparallelscan_function) (void);
+
+/* prepare for parallel index scan */
+typedef void (*aminitparallelscan_function) (void *target);
+
+/* (re)start parallel index scan */
+typedef void (*amparallelrescan_function) (IndexScanDesc scan);
 
 /*
  * API struct for an index AM.  Note this must be stored in a single palloc'd
@@ -196,6 +208,11 @@ typedef struct IndexAmRoutine
 	amendscan_function amendscan;
 	ammarkpos_function ammarkpos;		/* can be NULL */
 	amrestrpos_function amrestrpos;		/* can be NULL */
+
+	/* interface functions to support parallel index scans */
+	amestimateparallelscan_function amestimateparallelscan;		/* can be NULL */
+	aminitparallelscan_function aminitparallelscan;		/* can be NULL */
+	amparallelrescan_function amparallelrescan; /* can be NULL */
 } IndexAmRoutine;
 
 

src/include/access/genam.h

@@ -83,6 +83,8 @@ typedef bool (*IndexBulkDeleteCallback) (ItemPointer itemptr, void *state);
 typedef struct IndexScanDescData *IndexScanDesc;
 typedef struct SysScanDescData *SysScanDesc;
 
+typedef struct ParallelIndexScanDescData *ParallelIndexScanDesc;
+
 /*
  * Enumeration specifying the type of uniqueness check to perform in
  * index_insert().
@@ -144,6 +146,13 @@ 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 Size index_parallelscan_estimate(Relation indexrel, Snapshot snapshot);
+extern void index_parallelscan_initialize(Relation heaprel, Relation indexrel,
+							Snapshot snapshot, ParallelIndexScanDesc target);
+extern void index_parallelrescan(IndexScanDesc scan);
+extern IndexScanDesc index_beginscan_parallel(Relation heaprel,
+						 Relation indexrel, int nkeys, int norderbys,
+						 ParallelIndexScanDesc pscan);
 extern ItemPointer index_getnext_tid(IndexScanDesc scan,
 				  ScanDirection direction);
 extern HeapTuple index_fetch_heap(IndexScanDesc scan);

src/include/access/relscan.h

@@ -93,6 +93,7 @@ typedef struct IndexScanDescData
 	ScanKey		keyData;		/* array of index qualifier descriptors */
 	ScanKey		orderByData;	/* array of ordering op descriptors */
 	bool		xs_want_itup;	/* caller requests index tuples */
+	bool		xs_temp_snap;	/* unregister snapshot at scan end? */
 
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
@@ -126,8 +127,20 @@ typedef struct IndexScanDescData
 
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
+
+	/* parallel index scan information, in shared memory */
+	ParallelIndexScanDesc parallel_scan;
 }	IndexScanDescData;
 
+/* Generic structure for parallel scans */
+typedef struct ParallelIndexScanDescData
+{
+	Oid			ps_relid;
+	Oid			ps_indexid;
+	Size		ps_offset;		/* Offset in bytes of am specific structure */
+	char		ps_snapshot_data[FLEXIBLE_ARRAY_MEMBER];
+} ParallelIndexScanDescData;
+
 /* Struct for heap-or-index scans of system tables */
 typedef struct SysScanDescData
 {