pageinspect: Fix hash_bitmap_info not to read the underlying page.

It did that to verify that the page was an overflow page rather than
anything else, but that means that checking the status of all the
overflow bits requires reading the entire index.  So don't do that.
The new code validates that the page is not a primary bucket page
or bitmap page by looking at the metapage, so that using this on
large numbers of pages can be reasonably efficient.

Ashutosh Sharma, per a complaint from me, and with further
modifications by me.

1 files changed, 36 insertions, 26 deletions

diff --git a/contrib/pageinspect/hashfuncs.c b/contrib/pageinspect/hashfuncs.c
index 08663c109d1..88c807af5b2 100644
--- a/contrib/pageinspect/hashfuncs.c
+++ b/contrib/pageinspect/hashfuncs.c
@@ -380,21 +380,22 @@ hash_bitmap_info(PG_FUNCTION_ARGS)
 	Oid			indexRelid = PG_GETARG_OID(0);
 	uint64		ovflblkno = PG_GETARG_INT64(1);
 	HashMetaPage metap;
-	Buffer		buf,
-				metabuf;
+	Buffer		metabuf,
+				mapbuf;
 	BlockNumber bitmapblkno;
-	Page		page;
+	Page		mappage;
 	bool		bit = false;
-	HashPageOpaque	opaque;
 	TupleDesc	tupleDesc;
 	Relation	indexRel;
 	uint32		ovflbitno;
 	int32		bitmappage,
 				bitmapbit;
 	HeapTuple	tuple;
-	int			j;
+	int			i,
+				j;
 	Datum		values[3];
 	bool		nulls[3];
+	uint32	   *freep;
 
 	if (!superuser())
 		ereport(ERROR,
@@ -418,30 +419,30 @@ hash_bitmap_info(PG_FUNCTION_ARGS)
 				 errmsg("block number " UINT64_FORMAT " is out of range for relation \"%s\"",
 						ovflblkno, RelationGetRelationName(indexRel))));
 
-	buf = ReadBufferExtended(indexRel, MAIN_FORKNUM, (BlockNumber) ovflblkno,
-							 RBM_NORMAL, NULL);
-	LockBuffer(buf, BUFFER_LOCK_SHARE);
-	_hash_checkpage(indexRel, buf, LH_PAGE_TYPE);
-	page = BufferGetPage(buf);
-	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-
-	if (opaque->hasho_flag != LH_OVERFLOW_PAGE)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("page is not an overflow page"),
-				 errdetail("Expected %08x, got %08x.",
-							LH_OVERFLOW_PAGE, opaque->hasho_flag)));
-
-	if (BlockNumberIsValid(opaque->hasho_prevblkno))
-		bit = true;
-
-	UnlockReleaseBuffer(buf);
-
 	/* Read the metapage so we can determine which bitmap page to use */
 	metabuf = _hash_getbuf(indexRel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
 	metap = HashPageGetMeta(BufferGetPage(metabuf));
 
-	/* Identify overflow bit number */
+	/*
+	 * Reject attempt to read the bit for a metapage or bitmap page; this is
+	 * only meaningful for overflow pages.
+	 */
+	if (ovflblkno == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid overflow block number %u",
+						(BlockNumber) ovflblkno)));
+	for (i = 0; i < metap->hashm_nmaps; i++)
+		if (metap->hashm_mapp[i] == ovflblkno)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("invalid overflow block number %u",
+							(BlockNumber) ovflblkno)));
+
+	/*
+	 * Identify overflow bit number.  This will error out for primary bucket
+	 * pages, and we've already rejected the metapage and bitmap pages above.
+	 */
 	ovflbitno = _hash_ovflblkno_to_bitno(metap, (BlockNumber) ovflblkno);
 
 	bitmappage = ovflbitno >> BMPG_SHIFT(metap);
@@ -450,12 +451,21 @@ hash_bitmap_info(PG_FUNCTION_ARGS)
 	if (bitmappage >= metap->hashm_nmaps)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("invalid overflow bit number %u", ovflbitno)));
+				 errmsg("invalid overflow block number %u",
+						(BlockNumber) ovflblkno)));
 
 	bitmapblkno = metap->hashm_mapp[bitmappage];
 
 	_hash_relbuf(indexRel, metabuf);
 
+	/* Check the status of bitmap bit for overflow page */
+	mapbuf = _hash_getbuf(indexRel, bitmapblkno, HASH_READ, LH_BITMAP_PAGE);
+	mappage = BufferGetPage(mapbuf);
+	freep = HashPageGetBitmap(mappage);
+
+	bit = ISSET(freep, bitmapbit) != 0;
+
+	_hash_relbuf(indexRel, mapbuf);
 	index_close(indexRel, AccessShareLock);
 
 	/* Build a tuple descriptor for our result type */