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Introduce CompactAttribute array in TupleDesc

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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
This commit is contained in:
David Rowley
2024-12-03 16:50:59 +13:00
parent e4c8865196
commit d28dff3f6c
14 changed files with 246 additions and 68 deletions
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21
src/backend/utils/cache/relcache.c vendored
View File

@@ -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 */