pgstattuple: Add pgstathashindex.

Since pgstattuple v1.5 hasn't been released yet, no need for a new
extension version.  The new function exposes statistics about hash
indexes similar to what other pgstatindex functions return for other
index types.

Ashutosh Sharma, reviewed by Kuntal Ghosh.  Substantial further
revisions by me.

4 files changed, 223 insertions, 0 deletions

diff --git a/contrib/pgstattuple/expected/pgstattuple.out b/contrib/pgstattuple/expected/pgstattuple.out
index e920234488..169d1932b2 100644
--- a/contrib/pgstattuple/expected/pgstattuple.out
+++ b/contrib/pgstattuple/expected/pgstattuple.out
@@ -130,3 +130,11 @@ select * from pgstatginindex('test_ginidx');
        2 |             0 |              0
 (1 row)
 
+create index test_hashidx on test using hash (b);
+WARNING:  hash indexes are not WAL-logged and their use is discouraged
+select * from pgstathashindex('test_hashidx');
+ version | bucket_pages | overflow_pages | bitmap_pages | zero_pages | live_items | dead_items | free_percent 
+---------+--------------+----------------+--------------+------------+------------+------------+--------------
+       2 |            4 |              0 |            1 |          0 |          0 |          0 |          100
+(1 row)
+
diff --git a/contrib/pgstattuple/pgstatindex.c b/contrib/pgstattuple/pgstatindex.c
index b40669250a..17a53e3bb7 100644
--- a/contrib/pgstattuple/pgstatindex.c
+++ b/contrib/pgstattuple/pgstatindex.c
@@ -29,6 +29,7 @@
 
 #include "access/gin_private.h"
 #include "access/heapam.h"
+#include "access/hash.h"
 #include "access/htup_details.h"
 #include "access/nbtree.h"
 #include "catalog/namespace.h"
@@ -36,6 +37,7 @@
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
+#include "storage/lmgr.h"
 #include "utils/builtins.h"
 #include "utils/rel.h"
 #include "utils/varlena.h"
@@ -54,6 +56,7 @@ PG_FUNCTION_INFO_V1(pgstatindexbyid);
 PG_FUNCTION_INFO_V1(pg_relpages);
 PG_FUNCTION_INFO_V1(pg_relpagesbyid);
 PG_FUNCTION_INFO_V1(pgstatginindex);
+PG_FUNCTION_INFO_V1(pgstathashindex);
 
 PG_FUNCTION_INFO_V1(pgstatindex_v1_5);
 PG_FUNCTION_INFO_V1(pgstatindexbyid_v1_5);
@@ -66,6 +69,7 @@ Datum pgstatginindex_internal(Oid relid, FunctionCallInfo fcinfo);
 #define IS_INDEX(r) ((r)->rd_rel->relkind == RELKIND_INDEX)
 #define IS_BTREE(r) ((r)->rd_rel->relam == BTREE_AM_OID)
 #define IS_GIN(r) ((r)->rd_rel->relam == GIN_AM_OID)
+#define IS_HASH(r) ((r)->rd_rel->relam == HASH_AM_OID)
 
 /* ------------------------------------------------
  * A structure for a whole btree index statistics
@@ -102,7 +106,29 @@ typedef struct GinIndexStat
 	int64		pending_tuples;
 } GinIndexStat;
 
+/* ------------------------------------------------
+ * A structure for a whole HASH index statistics
+ * used by pgstathashindex().
+ * ------------------------------------------------
+ */
+typedef struct HashIndexStat
+{
+	int32	version;
+	int32	space_per_page;
+
+	BlockNumber	bucket_pages;
+	BlockNumber overflow_pages;
+	BlockNumber bitmap_pages;
+	BlockNumber zero_pages;
+
+	int64	live_items;
+	int64	dead_items;
+	uint64	free_space;
+} HashIndexStat;
+
 static Datum pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo);
+static void GetHashPageStats(Page page, HashIndexStat *stats);
+
 
 /* ------------------------------------------------------
  * pgstatindex()
@@ -528,3 +554,172 @@ pgstatginindex_internal(Oid relid, FunctionCallInfo fcinfo)
 
 	return (result);
 }
+
+/* ------------------------------------------------------
+ * pgstathashindex()
+ *
+ * Usage: SELECT * FROM pgstathashindex('hashindex');
+ * ------------------------------------------------------
+ */
+Datum
+pgstathashindex(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+	BlockNumber	nblocks;
+	BlockNumber	blkno;
+	Relation	rel;
+	HashIndexStat stats;
+	BufferAccessStrategy bstrategy;
+	HeapTuple	tuple;
+	TupleDesc	tupleDesc;
+	Datum		values[8];
+	bool		nulls[8];
+	Buffer		metabuf;
+	HashMetaPage	metap;
+	float8		free_percent;
+	uint64		total_space;
+
+	rel = index_open(relid, AccessShareLock);
+
+	if (!IS_HASH(rel))
+		elog(ERROR, "relation \"%s\" is not a HASH index",
+			 RelationGetRelationName(rel));
+
+	/*
+	 * Reject attempts to read non-local temporary relations; we would be
+	 * likely to get wrong data since we have no visibility into the owning
+	 * session's local buffers.
+	 */
+	if (RELATION_IS_OTHER_TEMP(rel))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			   errmsg("cannot access temporary indexes of other sessions")));
+
+	/* Get the information we need from the metapage. */
+	memset(&stats, 0, sizeof(stats));
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+	stats.version = metap->hashm_version;
+	stats.space_per_page = metap->hashm_bsize;
+	_hash_relbuf(rel, metabuf);
+
+	/* Get the current relation length */
+	nblocks = RelationGetNumberOfBlocks(rel);
+
+	/* prepare access strategy for this index */
+	bstrategy = GetAccessStrategy(BAS_BULKREAD);
+
+	/* Start from blkno 1 as 0th block is metapage */
+	for (blkno = 1; blkno < nblocks; blkno++)
+	{
+		Buffer		buf;
+		Page		page;
+		HashPageOpaque	opaque;
+
+		CHECK_FOR_INTERRUPTS();
+
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
+								 bstrategy);
+		LockBuffer(buf, BUFFER_LOCK_SHARE);
+		page = (Page) BufferGetPage(buf);
+
+		if (PageIsNew(page))
+			stats.zero_pages++;
+		else if (PageGetSpecialSize(page) !=
+				 MAXALIGN(sizeof(HashPageOpaqueData)))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" contains corrupted page at block %u",
+							RelationGetRelationName(rel),
+							BufferGetBlockNumber(buf))));
+		else
+		{
+			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+			if (opaque->hasho_flag & LH_BUCKET_PAGE)
+			{
+				stats.bucket_pages++;
+				GetHashPageStats(page, &stats);
+			}
+			else if (opaque->hasho_flag & LH_OVERFLOW_PAGE)
+			{
+				stats.overflow_pages++;
+				GetHashPageStats(page, &stats);
+			}
+			else if (opaque->hasho_flag & LH_BITMAP_PAGE)
+				stats.bitmap_pages++;
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+					errmsg("unexpected page type 0x%04X in HASH index \"%s\" block %u",
+							opaque->hasho_flag, RelationGetRelationName(rel),
+							BufferGetBlockNumber(buf))));
+		}
+		UnlockReleaseBuffer(buf);
+	}
+
+	/* Done accessing the index */
+	index_close(rel, AccessShareLock);
+
+	/* Count zero pages as free space. */
+	stats.free_space += stats.zero_pages * stats.space_per_page;
+
+	/*
+	 * Total space available for tuples excludes the metapage and the bitmap
+	 * pages.
+	 */
+	total_space = (nblocks - (stats.bitmap_pages + 1)) * stats.space_per_page;
+
+	if (total_space == 0)
+		free_percent = 0.0;
+	else
+		free_percent = 100.0 * stats.free_space / total_space;
+
+	/*
+	 * Build a tuple descriptor for our result type
+	 */
+	if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	tupleDesc = BlessTupleDesc(tupleDesc);
+
+	/*
+	 * Build and return the tuple
+	 */
+	MemSet(nulls, 0, sizeof(nulls));
+	values[0] = Int32GetDatum(stats.version);
+	values[1] = Int64GetDatum((int64) stats.bucket_pages);
+	values[2] = Int64GetDatum((int64) stats.overflow_pages);
+	values[3] = Int64GetDatum((int64) stats.bitmap_pages);
+	values[4] = Int64GetDatum((int64) stats.zero_pages);
+	values[5] = Int64GetDatum(stats.live_items);
+	values[6] = Int64GetDatum(stats.dead_items);
+	values[7] = Float8GetDatum(free_percent);
+	tuple = heap_form_tuple(tupleDesc, values, nulls);
+
+	PG_RETURN_DATUM(HeapTupleGetDatum(tuple));
+}
+
+/* -------------------------------------------------
+ * GetHashPageStatis()
+ *
+ * Collect statistics of single hash page
+ * -------------------------------------------------
+ */
+static void
+GetHashPageStats(Page page, HashIndexStat *stats)
+{
+	OffsetNumber maxoff = PageGetMaxOffsetNumber(page);
+	int off;
+
+	/* count live and dead tuples, and free space */
+	for (off = FirstOffsetNumber; off <= maxoff; off++)
+	{
+		ItemId      id = PageGetItemId(page, off);
+
+		if (!ItemIdIsDead(id))
+			stats->live_items++;
+		else
+			stats->dead_items++;
+	}
+	stats->free_space += PageGetExactFreeSpace(page);
+}
diff --git a/contrib/pgstattuple/pgstattuple--1.4--1.5.sql b/contrib/pgstattuple/pgstattuple--1.4--1.5.sql
index 65d7f19c2a..84e112e1c2 100644
--- a/contrib/pgstattuple/pgstattuple--1.4--1.5.sql
+++ b/contrib/pgstattuple/pgstattuple--1.4--1.5.sql
@@ -109,3 +109,19 @@ AS 'MODULE_PATHNAME', 'pgstattuple_approx_v1_5'
 LANGUAGE C STRICT PARALLEL SAFE;
 
 REVOKE EXECUTE ON FUNCTION pgstattuple_approx(regclass) FROM PUBLIC;
+
+/* New stuff in 1.5 begins here */
+
+CREATE OR REPLACE FUNCTION pgstathashindex(IN relname regclass,
+	OUT version INTEGER,
+	OUT bucket_pages BIGINT,
+	OUT overflow_pages BIGINT,
+	OUT bitmap_pages BIGINT,
+	OUT zero_pages BIGINT,
+	OUT live_items BIGINT,
+	OUT dead_items BIGINT,
+	OUT free_percent FLOAT8)
+AS 'MODULE_PATHNAME', 'pgstathashindex'
+LANGUAGE C STRICT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pgstathashindex(regclass) FROM PUBLIC;
diff --git a/contrib/pgstattuple/sql/pgstattuple.sql b/contrib/pgstattuple/sql/pgstattuple.sql
index d22c9f1c46..81fd5d693b 100644
--- a/contrib/pgstattuple/sql/pgstattuple.sql
+++ b/contrib/pgstattuple/sql/pgstattuple.sql
@@ -47,3 +47,7 @@ select pg_relpages(relname) from pg_class where relname = 'test_pkey';
 create index test_ginidx on test using gin (b);
 
 select * from pgstatginindex('test_ginidx');
+
+create index test_hashidx on test using hash (b);
+
+select * from pgstathashindex('test_hashidx');