Fix multiple bugs in contrib/pgstattuple's pgstatindex() function.

Dead or half-dead index leaf pages were incorrectly reported as live, as a
consequence of a code rearrangement I made (during a moment of severe brain
fade, evidently) in commit d287818eb514d431.

The index metapage was not counted in index_size, causing that result to
not agree with the actual index size on-disk.

Index root pages were not counted in internal_pages, which is inconsistent
compared to the case of a root that's also a leaf (one-page index), where
the root would be counted in leaf_pages.  Aside from that inconsistency,
this could lead to additional transient discrepancies between the reported
page counts and index_size, since it's possible for pgstatindex's scan to
see zero or multiple pages marked as BTP_ROOT, if the root moves due to
a split during the scan.  With these fixes, index_size will always be
exactly one page more than the sum of the displayed page counts.

Also, the index_size result was incorrectly documented as being measured in
pages; it's always been measured in bytes.  (While fixing that, I couldn't
resist doing some small additional wordsmithing on the pgstattuple docs.)

Including the metapage causes the reported index_size to not be zero for
an empty index.  To preserve the desired property that the pgstattuple
regression test results are platform-independent (ie, BLCKSZ configuration
independent), scale the index_size result in the regression tests.

The documentation issue was reported by Otsuka Kenji, and the inconsistent
root page counting by Peter Geoghegan; the other problems noted by me.
Back-patch to all supported branches, because this has been broken for
a long time.

4 files changed, 68 insertions, 47 deletions

diff --git a/contrib/pgstattuple/expected/pgstattuple.out b/contrib/pgstattuple/expected/pgstattuple.out
index d769f6d4943..e920234488e 100644
--- a/contrib/pgstattuple/expected/pgstattuple.out
+++ b/contrib/pgstattuple/expected/pgstattuple.out
@@ -41,40 +41,44 @@ select pgstattuple(relname) from pg_class where relname = 'test';
  (0,0,0,0,0,0,0,0,0)
 (1 row)
 
-select * from pgstatindex('test_pkey');
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey');
  version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation 
 ---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
-       2 |          0 |          0 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
+       2 |          0 |          1 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
 (1 row)
 
-select * from pgstatindex('test_pkey'::text);
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::text);
  version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation 
 ---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
-       2 |          0 |          0 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
+       2 |          0 |          1 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
 (1 row)
 
-select * from pgstatindex('test_pkey'::name);
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::name);
  version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation 
 ---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
-       2 |          0 |          0 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
+       2 |          0 |          1 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
 (1 row)
 
-select * from pgstatindex('test_pkey'::regclass);
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::regclass);
  version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation 
 ---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
-       2 |          0 |          0 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
-(1 row)
-
-select pgstatindex(oid) from pg_class where relname = 'test_pkey';
-        pgstatindex        
----------------------------
- (2,0,0,0,0,0,0,0,NaN,NaN)
-(1 row)
-
-select pgstatindex(relname) from pg_class where relname = 'test_pkey';
-        pgstatindex        
----------------------------
- (2,0,0,0,0,0,0,0,NaN,NaN)
+       2 |          0 |          1 |             0 |              0 |          0 |           0 |             0 |              NaN |                NaN
 (1 row)
 
 select pg_relpages('test');
diff --git a/contrib/pgstattuple/pgstatindex.c b/contrib/pgstattuple/pgstatindex.c
index 8048a2cbe7c..9f1377c4cd9 100644
--- a/contrib/pgstattuple/pgstatindex.c
+++ b/contrib/pgstattuple/pgstatindex.c
@@ -79,7 +79,6 @@ typedef struct BTIndexStat
 	uint32		level;
 	BlockNumber root_blkno;
 
-	uint64		root_pages;
 	uint64		internal_pages;
 	uint64		leaf_pages;
 	uint64		empty_pages;
@@ -185,7 +184,6 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 	}
 
 	/* -- init counters -- */
-	indexStat.root_pages = 0;
 	indexStat.internal_pages = 0;
 	indexStat.leaf_pages = 0;
 	indexStat.empty_pages = 0;
@@ -218,7 +216,11 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 
 		/* Determine page type, and update totals */
 
-		if (P_ISLEAF(opaque))
+		if (P_ISDELETED(opaque))
+			indexStat.deleted_pages++;
+		else if (P_IGNORE(opaque))
+			indexStat.empty_pages++;	/* this is the "half dead" state */
+		else if (P_ISLEAF(opaque))
 		{
 			int			max_avail;
 
@@ -235,12 +237,6 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 			if (opaque->btpo_next != P_NONE && opaque->btpo_next < blkno)
 				indexStat.fragments++;
 		}
-		else if (P_ISDELETED(opaque))
-			indexStat.deleted_pages++;
-		else if (P_IGNORE(opaque))
-			indexStat.empty_pages++;
-		else if (P_ISROOT(opaque))
-			indexStat.root_pages++;
 		else
 			indexStat.internal_pages++;
 
@@ -269,7 +265,7 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 		values[j++] = psprintf("%d", indexStat.version);
 		values[j++] = psprintf("%d", indexStat.level);
 		values[j++] = psprintf(INT64_FORMAT,
-							   (indexStat.root_pages +
+							   (1 +		/* include the metapage in index_size */
 								indexStat.leaf_pages +
 								indexStat.internal_pages +
 								indexStat.deleted_pages +
diff --git a/contrib/pgstattuple/sql/pgstattuple.sql b/contrib/pgstattuple/sql/pgstattuple.sql
index 0e0ad0e15d5..d22c9f1c46d 100644
--- a/contrib/pgstattuple/sql/pgstattuple.sql
+++ b/contrib/pgstattuple/sql/pgstattuple.sql
@@ -15,12 +15,26 @@ select * from pgstattuple('test'::regclass);
 select pgstattuple(oid) from pg_class where relname = 'test';
 select pgstattuple(relname) from pg_class where relname = 'test';
 
-select * from pgstatindex('test_pkey');
-select * from pgstatindex('test_pkey'::text);
-select * from pgstatindex('test_pkey'::name);
-select * from pgstatindex('test_pkey'::regclass);
-select pgstatindex(oid) from pg_class where relname = 'test_pkey';
-select pgstatindex(relname) from pg_class where relname = 'test_pkey';
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey');
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::text);
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::name);
+select version, tree_level,
+    index_size / current_setting('block_size')::int as index_size,
+    root_block_no, internal_pages, leaf_pages, empty_pages, deleted_pages,
+    avg_leaf_density, leaf_fragmentation
+    from pgstatindex('test_pkey'::regclass);
 
 select pg_relpages('test');
 select pg_relpages('test_pkey');
diff --git a/doc/src/sgml/pgstattuple.sgml b/doc/src/sgml/pgstattuple.sgml
index 18d244ba3a3..61340bedbc3 100644
--- a/doc/src/sgml/pgstattuple.sgml
+++ b/doc/src/sgml/pgstattuple.sgml
@@ -130,9 +130,9 @@ free_percent       | 1.95
     <listitem>
      <para>
       This is the same as <function>pgstattuple(regclass)</function>, except
-      that the target relation is specified by TEXT. This function is kept
+      that the target relation is specified as TEXT. This function is kept
       because of backward-compatibility so far, and will be deprecated in
-      the future release.
+      some future release.
      </para>
     </listitem>
    </varlistentry>
@@ -154,13 +154,13 @@ test=&gt; SELECT * FROM pgstatindex('pg_cast_oid_index');
 -[ RECORD 1 ]------+------
 version            | 2
 tree_level         | 0
-index_size         | 8192
+index_size         | 16384
 root_block_no      | 1
 internal_pages     | 0
 leaf_pages         | 1
 empty_pages        | 0
 deleted_pages      | 0
-avg_leaf_density   | 50.27
+avg_leaf_density   | 54.27
 leaf_fragmentation | 0
 </programlisting>
      </para>
@@ -194,13 +194,13 @@ leaf_fragmentation | 0
        <row>
         <entry><structfield>index_size</structfield></entry>
         <entry><type>bigint</type></entry>
-        <entry>Total number of pages in index</entry>
+        <entry>Total index size in bytes</entry>
        </row>
 
        <row>
         <entry><structfield>root_block_no</structfield></entry>
         <entry><type>bigint</type></entry>
-        <entry>Location of root block</entry>
+        <entry>Location of root page (zero if none)</entry>
        </row>
 
        <row>
@@ -245,6 +245,13 @@ leaf_fragmentation | 0
     </para>
 
     <para>
+     The reported <literal>index_size</> will normally correspond to one more
+     page than is accounted for by <literal>internal_pages + leaf_pages +
+     empty_pages + deleted_pages</literal>, because it also includes the
+     index's metapage.
+    </para>
+
+    <para>
      As with <function>pgstattuple</>, the results are accumulated
      page-by-page, and should not be expected to represent an
      instantaneous snapshot of the whole index.
@@ -260,9 +267,9 @@ leaf_fragmentation | 0
     <listitem>
      <para>
       This is the same as <function>pgstatindex(regclass)</function>, except
-      that the target index is specified by TEXT. This function is kept
+      that the target index is specified as TEXT. This function is kept
       because of backward-compatibility so far, and will be deprecated in
-      the future release.
+      some future release.
      </para>
     </listitem>
    </varlistentry>
@@ -351,9 +358,9 @@ pending_tuples | 0
     <listitem>
      <para>
       This is the same as <function>pg_relpages(regclass)</function>, except
-      that the target relation is specified by TEXT. This function is kept
+      that the target relation is specified as TEXT. This function is kept
       because of backward-compatibility so far, and will be deprecated in
-      the future release.
+      some future release.
      </para>
     </listitem>
    </varlistentry>
@@ -370,7 +377,7 @@ pending_tuples | 0
      <para>
       <function>pgstattuple_approx</function> is a faster alternative to
       <function>pgstattuple</function> that returns approximate results.
-      The argument is the target relation's OID.
+      The argument is the target relation's name or OID.
       For example:
 <programlisting>
 test=&gt; SELECT * FROM pgstattuple_approx('pg_catalog.pg_proc'::regclass);