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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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70
src/backend/access/common/tupdesc.c
View File

@@ -56,6 +56,33 @@ ResourceOwnerForgetTupleDesc(ResourceOwner owner, TupleDesc tupdesc)
ResourceOwnerForget(owner, PointerGetDatum(tupdesc), &tupdesc_resowner_desc);
}
/*
* populate_compact_attribute
* Fills in the corresponding CompactAttribute element from the
* Form_pg_attribute for the given attribute number. This must be called
* whenever a change is made to a Form_pg_attribute in the TupleDesc.
*/
void
populate_compact_attribute(TupleDesc tupdesc, int attnum)
{
Form_pg_attribute src = TupleDescAttr(tupdesc, attnum);
CompactAttribute *dst = &tupdesc->compact_attrs[attnum];
memset(dst, 0, sizeof(CompactAttribute));
dst->attcacheoff = -1;
dst->attlen = src->attlen;
dst->attbyval = src->attbyval;
dst->attispackable = (src->attstorage != TYPSTORAGE_PLAIN);
dst->atthasmissing = src->atthasmissing;
dst->attisdropped = src->attisdropped;
dst->attgenerated = (src->attgenerated != '\0');
dst->attnotnull = src->attnotnull;
dst->attalign = src->attalign;
}
/*
* CreateTemplateTupleDesc
* This function allocates an empty tuple descriptor structure.
@@ -74,18 +101,19 @@ CreateTemplateTupleDesc(int natts)
Assert(natts >= 0);
/*
* Allocate enough memory for the tuple descriptor, including the
* attribute rows.
* Allocate enough memory for the tuple descriptor, the CompactAttribute
* array and also an array of the full FormData_pg_attribute data.
*
* Note: the attribute array stride is sizeof(FormData_pg_attribute),
* since we declare the array elements as FormData_pg_attribute for
* notational convenience. However, we only guarantee that the first
* Note: the 'attrs' array stride is sizeof(FormData_pg_attribute), since
* we declare the array elements as FormData_pg_attribute for notational
* convenience. However, we only guarantee that the first
* ATTRIBUTE_FIXED_PART_SIZE bytes of each entry are valid; most code that
* copies tupdesc entries around copies just that much. In principle that
* could be less due to trailing padding, although with the current
* definition of pg_attribute there probably isn't any padding.
*/
desc = (TupleDesc) palloc(offsetof(struct TupleDescData, attrs) +
desc = (TupleDesc) palloc(offsetof(struct TupleDescData, compact_attrs) +
natts * sizeof(CompactAttribute) +
natts * sizeof(FormData_pg_attribute));
/*
@@ -96,6 +124,7 @@ CreateTemplateTupleDesc(int natts)
desc->tdtypeid = RECORDOID;
desc->tdtypmod = -1;
desc->tdrefcount = -1; /* assume not reference-counted */
desc->attrs = TupleDescAttrAddress(desc);
return desc;
}
@@ -117,8 +146,10 @@ CreateTupleDesc(int natts, Form_pg_attribute *attrs)
desc = CreateTemplateTupleDesc(natts);
for (i = 0; i < natts; ++i)
{
memcpy(TupleDescAttr(desc, i), attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
populate_compact_attribute(desc, i);
}
return desc;
}
@@ -155,6 +186,8 @@ CreateTupleDescCopy(TupleDesc tupdesc)
att->atthasmissing = false;
att->attidentity = '\0';
att->attgenerated = '\0';
populate_compact_attribute(desc, i);
}
/* We can copy the tuple type identification, too */
@@ -183,6 +216,9 @@ CreateTupleDescCopyConstr(TupleDesc tupdesc)
TupleDescAttr(tupdesc, 0),
desc->natts * sizeof(FormData_pg_attribute));
for (i = 0; i < desc->natts; i++)
populate_compact_attribute(desc, i);
/* Copy the TupleConstr data structure, if any */
if (constr)
{
@@ -207,7 +243,7 @@ CreateTupleDescCopyConstr(TupleDesc tupdesc)
{
if (constr->missing[i].am_present)
{
Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
CompactAttribute *attr = TupleDescCompactAttr(tupdesc, i);
cpy->missing[i].am_value = datumCopy(constr->missing[i].am_value,
attr->attbyval,
@@ -252,9 +288,15 @@ TupleDescCopy(TupleDesc dst, TupleDesc src)
{
int i;
/* Flat-copy the header and attribute array */
/* Flat-copy the header and attribute arrays */
memcpy(dst, src, TupleDescSize(src));
/*
* Adjust 'attrs' to point to the dst FormData_pg_attribute array rather
* than the src's.
*/
dst->attrs = TupleDescAttrAddress(dst);
/*
* Since we're not copying constraints and defaults, clear fields
* associated with them.
@@ -268,6 +310,8 @@ TupleDescCopy(TupleDesc dst, TupleDesc src)
att->atthasmissing = false;
att->attidentity = '\0';
att->attgenerated = '\0';
populate_compact_attribute(dst, i);
}
dst->constr = NULL;
@@ -322,6 +366,8 @@ TupleDescCopyEntry(TupleDesc dst, AttrNumber dstAttno,
dstAtt->atthasmissing = false;
dstAtt->attidentity = '\0';
dstAtt->attgenerated = '\0';
populate_compact_attribute(dst, dstAttno - 1);
}
/*
@@ -521,7 +567,7 @@ equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2)
return false;
if (missval1->am_present)
{
Form_pg_attribute missatt1 = TupleDescAttr(tupdesc1, i);
CompactAttribute *missatt1 = TupleDescCompactAttr(tupdesc1, i);
if (!datumIsEqual(missval1->am_value, missval2->am_value,
missatt1->attbyval, missatt1->attlen))
@@ -714,6 +760,8 @@ TupleDescInitEntry(TupleDesc desc,
att->attcompression = InvalidCompressionMethod;
att->attcollation = typeForm->typcollation;
populate_compact_attribute(desc, attributeNumber - 1);
ReleaseSysCache(tuple);
}
@@ -821,6 +869,8 @@ TupleDescInitBuiltinEntry(TupleDesc desc,
default:
elog(ERROR, "unsupported type %u", oidtypeid);
}
populate_compact_attribute(desc, attributeNumber - 1);
}
/*