Remove forced toast recompression in VACUUM FULL/CLUSTER

The extra checks added by the recompression of toast data introduced in
bbe0a81 is proving to have a performance impact on VACUUM or CLUSTER
even if no recompression is done.  This is more noticeable with more
toastable columns that contain non-NULL values.

Improvements could be done to make those extra checks less expensive,
but that's not material for 14 at this stage, and we are not sure either
if the code path of VACUUM FULL/CLUSTER is adapted for this job.

Per discussion with several people, including Andres Freund, Robert
Haas, Álvaro Herrera, Tom Lane and myself.

Discussion: https://postgr.es/m/20210527003144.xxqppojoiwurc2iz@alap3.anarazel.de

1 files changed, 1 insertions, 60 deletions

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index e2cd79ec546..1b8b640012a 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -19,7 +19,6 @@
  */
 #include "postgres.h"
 
-#include "access/detoast.h"
 #include "access/genam.h"
 #include "access/heapam.h"
 #include "access/heaptoast.h"
@@ -27,7 +26,6 @@
 #include "access/rewriteheap.h"
 #include "access/syncscan.h"
 #include "access/tableam.h"
-#include "access/toast_compression.h"
 #include "access/tsmapi.h"
 #include "access/xact.h"
 #include "catalog/catalog.h"
@@ -2463,64 +2461,14 @@ reform_and_rewrite_tuple(HeapTuple tuple,
 	TupleDesc	newTupDesc = RelationGetDescr(NewHeap);
 	HeapTuple	copiedTuple;
 	int			i;
-	bool		values_free[MaxTupleAttributeNumber];
-
-	memset(values_free, 0, newTupDesc->natts * sizeof(bool));
 
 	heap_deform_tuple(tuple, oldTupDesc, values, isnull);
 
+	/* Be sure to null out any dropped columns */
 	for (i = 0; i < newTupDesc->natts; i++)
 	{
-		/* Be sure to null out any dropped columns */
 		if (TupleDescAttr(newTupDesc, i)->attisdropped)
 			isnull[i] = true;
-		else if (!isnull[i] && TupleDescAttr(newTupDesc, i)->attlen == -1)
-		{
-			/*
-			 * Use this opportunity to force recompression of any data that's
-			 * compressed with some TOAST compression method other than the
-			 * one configured for the column.  We don't actually need to
-			 * perform the compression here; we just need to decompress.  That
-			 * will trigger recompression later on.
-			 */
-			struct varlena *new_value;
-			ToastCompressionId cmid;
-			char		cmethod;
-			char		targetmethod;
-
-			new_value = (struct varlena *) DatumGetPointer(values[i]);
-			cmid = toast_get_compression_id(new_value);
-
-			/* nothing to be done for uncompressed data */
-			if (cmid == TOAST_INVALID_COMPRESSION_ID)
-				continue;
-
-			/* convert existing compression id to compression method */
-			switch (cmid)
-			{
-				case TOAST_PGLZ_COMPRESSION_ID:
-					cmethod = TOAST_PGLZ_COMPRESSION;
-					break;
-				case TOAST_LZ4_COMPRESSION_ID:
-					cmethod = TOAST_LZ4_COMPRESSION;
-					break;
-				default:
-					elog(ERROR, "invalid compression method id %d", cmid);
-					cmethod = '\0'; /* keep compiler quiet */
-			}
-
-			/* figure out what the target method is */
-			targetmethod = TupleDescAttr(newTupDesc, i)->attcompression;
-			if (!CompressionMethodIsValid(targetmethod))
-				targetmethod = default_toast_compression;
-
-			/* if compression method doesn't match then detoast the value */
-			if (targetmethod != cmethod)
-			{
-				values[i] = PointerGetDatum(detoast_attr(new_value));
-				values_free[i] = true;
-			}
-		}
 	}
 
 	copiedTuple = heap_form_tuple(newTupDesc, values, isnull);
@@ -2528,13 +2476,6 @@ reform_and_rewrite_tuple(HeapTuple tuple,
 	/* The heap rewrite module does the rest */
 	rewrite_heap_tuple(rwstate, tuple, copiedTuple);
 
-	/* Free any value detoasted previously */
-	for (i = 0; i < newTupDesc->natts; i++)
-	{
-		if (values_free[i])
-			pfree(DatumGetPointer(values[i]));
-	}
-
 	heap_freetuple(copiedTuple);
 }