Consistently use PageGetExactFreeSpace() in pgstattuple.

tglsfdc · tglsfdc · commit de239d01e7cc · 2024-09-09T14:34:10.000-04:00
Previously this code used PageGetHeapFreeSpace on heap pages, and usually used PageGetFreeSpace on index pages (though for some reason GetHashPageStats used PageGetExactFreeSpace instead). The difference is that those functions subtract off the size of a line pointer, and PageGetHeapFreeSpace has some additional rules about returning zero if adding another line pointer would require exceeding MaxHeapTuplesPerPage. Those things make sense when testing to see if a new tuple can be put on the page, but they seem pretty strange for pure statistics collection. Additionally, statapprox_heap had a special rule about counting a "new" page as being fully available space. This also seems strange, because it's not actually usable until VACUUM or some such process initializes the page. Moreover, it's inconsistent with what pgstat_heap does, which is to count such a page as having zero free space. So make it work like pgstat_heap, which as of this patch unconditionally calls PageGetExactFreeSpace. This is more of a definitional change than a bug fix, so no back-patch. The module's documentation doesn't define exactly what "free space" means either, so we left that as-is. Frédéric Yhuel, reviewed by Rafia Sabih and Andreas Karlsson. Discussion: https://postgr.es/m/3a18f843-76f6-4a84-8cca-49537fefa15d@dalibo.com
diff --git a/contrib/pgstattuple/pgstatapprox.c b/contrib/pgstattuple/pgstatapprox.c
@@ -106,14 +106,7 @@ statapprox_heap(Relation rel, output_type *stat)
 
 		page = BufferGetPage(buf);
 
-		/*
-		 * It's not safe to call PageGetHeapFreeSpace() on new pages, so we
-		 * treat them as being free space for our purposes.
-		 */
-		if (!PageIsNew(page))
-			stat->free_space += PageGetHeapFreeSpace(page);
-		else
-			stat->free_space += BLCKSZ - SizeOfPageHeaderData;
+		stat->free_space += PageGetExactFreeSpace(page);
 
 		/* We may count the page as scanned even if it's new/empty */
 		scanned++;
diff --git a/contrib/pgstattuple/pgstatindex.c b/contrib/pgstattuple/pgstatindex.c
@@ -311,7 +311,7 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 
 			max_avail = BLCKSZ - (BLCKSZ - ((PageHeader) page)->pd_special + SizeOfPageHeaderData);
 			indexStat.max_avail += max_avail;
-			indexStat.free_space += PageGetFreeSpace(page);
+			indexStat.free_space += PageGetExactFreeSpace(page);
 
 			indexStat.leaf_pages++;
 
diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
@@ -372,7 +372,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 			buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,
 										RBM_NORMAL, hscan->rs_strategy);
 			LockBuffer(buffer, BUFFER_LOCK_SHARE);
-			stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));
+			stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));
 			UnlockReleaseBuffer(buffer);
 			block++;
 		}
@@ -385,7 +385,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 		buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,
 									RBM_NORMAL, hscan->rs_strategy);
 		LockBuffer(buffer, BUFFER_LOCK_SHARE);
-		stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));
+		stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));
 		UnlockReleaseBuffer(buffer);
 		block++;
 	}
@@ -565,7 +565,7 @@ pgstat_index_page(pgstattuple_type *stat, Page page,
 {
 	OffsetNumber i;
 
-	stat->free_space += PageGetFreeSpace(page);
+	stat->free_space += PageGetExactFreeSpace(page);
 
 	for (i = minoff; i <= maxoff; i = OffsetNumberNext(i))
 	{

Original file line number	Diff line number	Diff line change
`@@ -372,7 +372,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)`
`372`	`372`	`buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,`
`373`	`373`	`RBM_NORMAL, hscan->rs_strategy);`
`374`	`374`	`LockBuffer(buffer, BUFFER_LOCK_SHARE);`
`375`		`- stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));`
	`375`	`+ stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));`
`376`	`376`	`UnlockReleaseBuffer(buffer);`
`377`	`377`	`block++;`
`378`	`378`	`}`
`@@ -385,7 +385,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)`
`385`	`385`	`buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,`
`386`	`386`	`RBM_NORMAL, hscan->rs_strategy);`
`387`	`387`	`LockBuffer(buffer, BUFFER_LOCK_SHARE);`
`388`		`- stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));`
	`388`	`+ stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));`
`389`	`389`	`UnlockReleaseBuffer(buffer);`
`390`	`390`	`block++;`
`391`	`391`	`}`
`@@ -565,7 +565,7 @@ pgstat_index_page(pgstattuple_type *stat, Page page,`
`565`	`565`	`{`
`566`	`566`	`OffsetNumber i;`
`567`	`567`
`568`		`- stat->free_space += PageGetFreeSpace(page);`
	`568`	`+ stat->free_space += PageGetExactFreeSpace(page);`
`569`	`569`
`570`	`570`	`for (i = minoff; i <= maxoff; i = OffsetNumberNext(i))`
`571`	`571`	`{`