Introduce CompactAttribute array in TupleDesc

The new compact_attrs array stores a few select fields from
FormData_pg_attribute in a more compact way, using only 16 bytes per
column instead of the 104 bytes that FormData_pg_attribute uses.  Using
CompactAttribute allows performance-critical operations such as tuple
deformation to be performed without looking at the FormData_pg_attribute
element in TupleDesc which means fewer cacheline accesses.  With this
change, NAMEDATALEN could be increased with a much smaller negative impact
on performance.

For some workloads, tuple deformation can be the most CPU intensive part
of processing the query.  Some testing with 16 columns on a table
where the first column is variable length showed around a 10% increase in
transactions per second for an OLAP type query performing aggregation on
the 16th column.  However, in certain cases, the increases were much
higher, up to ~25% on one AMD Zen4 machine.

This also makes pg_attribute.attcacheoff redundant.  A follow-on commit
will remove it, thus shrinking the FormData_pg_attribute struct by 4
bytes.

Author: David Rowley
Discussion: https://postgr.es/m/CAApHDvrBztXP3yx=NKNmo3xwFAFhEdyPnvrDg3=M0RhDs+4vYw@mail.gmail.com
Reviewed-by: Andres Freund, Victor Yegorov

1 files changed, 15 insertions, 6 deletions

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index d0892cee24d..e230450d2c3 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -585,6 +585,8 @@ RelationBuildTupleDesc(Relation relation)
 			   attp,
 			   ATTRIBUTE_FIXED_PART_SIZE);
 
+		populate_compact_attribute(relation->rd_att, attnum - 1);
+
 		/* Update constraint/default info */
 		if (attp->attnotnull)
 			constr->has_not_null = true;
@@ -674,12 +676,12 @@ RelationBuildTupleDesc(Relation relation)
 #endif
 
 	/*
-	 * However, we can easily set the attcacheoff value for the first
-	 * attribute: it must be zero.  This eliminates the need for special cases
-	 * for attnum=1 that used to exist in fastgetattr() and index_getattr().
+	 * We can easily set the attcacheoff value for the first attribute: it
+	 * must be zero.  This eliminates the need for special cases for attnum=1
+	 * that used to exist in fastgetattr() and index_getattr().
 	 */
 	if (RelationGetNumberOfAttributes(relation) > 0)
-		TupleDescAttr(relation->rd_att, 0)->attcacheoff = 0;
+		TupleDescCompactAttr(relation->rd_att, 0)->attcacheoff = 0;
 
 	/*
 	 * Set up constraint/default info
@@ -1965,10 +1967,12 @@ formrdesc(const char *relationName, Oid relationReltype,
 		has_not_null |= attrs[i].attnotnull;
 		/* make sure attcacheoff is valid */
 		TupleDescAttr(relation->rd_att, i)->attcacheoff = -1;
+
+		populate_compact_attribute(relation->rd_att, i);
 	}
 
 	/* initialize first attribute's attcacheoff, cf RelationBuildTupleDesc */
-	TupleDescAttr(relation->rd_att, 0)->attcacheoff = 0;
+	TupleDescCompactAttr(relation->rd_att, 0)->attcacheoff = 0;
 
 	/* mark not-null status */
 	if (has_not_null)
@@ -3579,6 +3583,7 @@ RelationBuildLocalRelation(const char *relname,
 		datt->attgenerated = satt->attgenerated;
 		datt->attnotnull = satt->attnotnull;
 		has_not_null |= satt->attnotnull;
+		populate_compact_attribute(rel->rd_att, i);
 	}
 
 	if (has_not_null)
@@ -4399,10 +4404,12 @@ BuildHardcodedDescriptor(int natts, const FormData_pg_attribute *attrs)
 		memcpy(TupleDescAttr(result, i), &attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
 		/* make sure attcacheoff is valid */
 		TupleDescAttr(result, i)->attcacheoff = -1;
+
+		populate_compact_attribute(result, i);
 	}
 
 	/* initialize first attribute's attcacheoff, cf RelationBuildTupleDesc */
-	TupleDescAttr(result, 0)->attcacheoff = 0;
+	TupleDescCompactAttr(result, 0)->attcacheoff = 0;
 
 	/* Note: we don't bother to set up a TupleConstr entry */
 
@@ -6166,6 +6173,8 @@ load_relcache_init_file(bool shared)
 				goto read_failed;
 
 			has_not_null |= attr->attnotnull;
+
+			populate_compact_attribute(rel->rd_att, i);
 		}
 
 		/* next read the access method specific field */