Fix pgstattuple's handling of unused hash pages.

Hash indexes can contain both pages which are all-zeroes (i.e.
PageIsNew()) and pages which have been initialized but currently
aren't used.  The latter category can happen either when a page
has been reserved but not yet used or when it is used for a time
and then freed.  pgstattuple was only prepared to deal with the
pages that are actually-zeroes, which it called zero_pages.
Rename the column to unused_pages (extension version 1.5 is
as-yet-unreleased) and make it count both kinds of unused pages.

Along the way, slightly tidy up the way we test for pages of
various types.

Robert Haas and Ashutosh Sharma, reviewed by Amit Kapila

Discussion: http://postgr.es/m/CAE9k0PkTtKFB3YndOyQMjwuHx+-FtUP1ynK8E-nHtetoow3NtQ@mail.gmail.com

contrib/pgstattuple/expected/pgstattuple.out

@@ -132,9 +132,9 @@ select * from pgstatginindex('test_ginidx');
 
 create index test_hashidx on test using hash (b);
 select * from pgstathashindex('test_hashidx');
- version | bucket_pages | overflow_pages | bitmap_pages | zero_pages | live_items | dead_items | free_percent 
----------+--------------+----------------+--------------+------------+------------+------------+--------------
-       3 |            4 |              0 |            1 |          0 |          0 |          0 |          100
+ version | bucket_pages | overflow_pages | bitmap_pages | unused_pages | live_items | dead_items | free_percent 
+---------+--------------+----------------+--------------+--------------+------------+------------+--------------
+       3 |            4 |              0 |            1 |            0 |          0 |          0 |          100
 (1 row)
 
 -- these should error with the wrong type

contrib/pgstattuple/pgstatindex.c

@@ -119,7 +119,7 @@ typedef struct HashIndexStat
 	BlockNumber	bucket_pages;
 	BlockNumber overflow_pages;
 	BlockNumber bitmap_pages;
-	BlockNumber zero_pages;
+	BlockNumber unused_pages;
 
 	int64	live_items;
 	int64	dead_items;
@@ -634,7 +634,6 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	{
 		Buffer		buf;
 		Page		page;
-		HashPageOpaque	opaque;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -644,7 +643,7 @@ pgstathashindex(PG_FUNCTION_ARGS)
 		page = (Page) BufferGetPage(buf);
 
 		if (PageIsNew(page))
-			stats.zero_pages++;
+			stats.unused_pages++;
 		else if (PageGetSpecialSize(page) !=
 				 MAXALIGN(sizeof(HashPageOpaqueData)))
 			ereport(ERROR,
@@ -654,19 +653,26 @@ pgstathashindex(PG_FUNCTION_ARGS)
 							BufferGetBlockNumber(buf))));
 		else
 		{
+			HashPageOpaque	opaque;
+			int		pagetype;
+
 			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			if (opaque->hasho_flag & LH_BUCKET_PAGE)
+			pagetype = opaque->hasho_flag & LH_PAGE_TYPE;
+
+			if (pagetype == LH_BUCKET_PAGE)
 			{
 				stats.bucket_pages++;
 				GetHashPageStats(page, &stats);
 			}
-			else if (opaque->hasho_flag & LH_OVERFLOW_PAGE)
+			else if (pagetype == LH_OVERFLOW_PAGE)
 			{
 				stats.overflow_pages++;
 				GetHashPageStats(page, &stats);
 			}
-			else if (opaque->hasho_flag & LH_BITMAP_PAGE)
+			else if (pagetype == LH_BITMAP_PAGE)
 				stats.bitmap_pages++;
+			else if (pagetype == LH_UNUSED_PAGE)
+				stats.unused_pages++;
 			else
 				ereport(ERROR,
 						(errcode(ERRCODE_INDEX_CORRUPTED),
@@ -680,8 +686,8 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	/* Done accessing the index */
 	index_close(rel, AccessShareLock);
 
-	/* Count zero pages as free space. */
-	stats.free_space += stats.zero_pages * stats.space_per_page;
+	/* Count unused pages as free space. */
+	stats.free_space += stats.unused_pages * stats.space_per_page;
 
 	/*
 	 * Total space available for tuples excludes the metapage and the bitmap
@@ -710,7 +716,7 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	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[4] = Int64GetDatum((int64) stats.unused_pages);
 	values[5] = Int64GetDatum(stats.live_items);
 	values[6] = Int64GetDatum(stats.dead_items);
 	values[7] = Float8GetDatum(free_percent);

contrib/pgstattuple/pgstattuple--1.4--1.5.sql

@@ -125,7 +125,7 @@ CREATE OR REPLACE FUNCTION pgstathashindex(IN relname regclass,
 	OUT bucket_pages BIGINT,
 	OUT overflow_pages BIGINT,
 	OUT bitmap_pages BIGINT,
-	OUT zero_pages BIGINT,
+	OUT unused_pages BIGINT,
 	OUT live_items BIGINT,
 	OUT dead_items BIGINT,
 	OUT free_percent FLOAT8)

doc/src/sgml/pgstattuple.sgml

@@ -372,7 +372,7 @@ version        | 2
 bucket_pages   | 33081
 overflow_pages | 0
 bitmap_pages   | 1
-zero_pages     | 32455
+unused_pages   | 32455
 live_items     | 10204006
 dead_items     | 0
 free_percent   | 61.8005949100872
@@ -418,9 +418,9 @@ free_percent   | 61.8005949100872
        </row>
 
        <row>
-        <entry><structfield>zero_pages</structfield></entry>
+        <entry><structfield>unused_pages</structfield></entry>
         <entry><type>bigint</type></entry>
-        <entry>Number of new or zero pages</entry>
+        <entry>Number of unused pages</entry>
        </row>
 
        <row>