Improve performance of index-only scans with many index columns.

StoreIndexTuple was a loop over index_getattr, which is O(N^2)
if the index columns are variable-width, and the performance
impact is already quite visible at ten columns.  The obvious
move is to replace that with a call to index_deform_tuple ...
but that's *also* a loop over index_getattr.  Improve it to
be essentially a clone of heap_deform_tuple.

(There are a few other places that loop over all index columns
with index_getattr, and perhaps should be changed likewise,
but most of them don't seem performance-critical.  Anyway, the
rest would mostly only be interested in the index key columns,
which there aren't likely to be so many of.  Wide index tuples
are a new thing with INCLUDE.)

Konstantin Knizhnik

Discussion: https://postgr.es/m/e06b2d27-04fc-5c0e-bb8c-ecd72aa24959@postgrespro.ru

diff --git a/src/backend/access/common/indextuple.c b/src/backend/access/common/indextuple.c
index 6a22b172036bab1c65fb6338c8075ed8922110b9..32c0ebb93a4a27c2ffc82afcc8ca85cd0d4255d5 100644 (file)
--- a/src/backend/access/common/indextuple.c
+++ b/src/backend/access/common/indextuple.c
@@ -418,19 +418,80 @@ nocache_index_getattr(IndexTuple tup,
  *
  * The caller must allocate sufficient storage for the output arrays.
  * (INDEX_MAX_KEYS entries should be enough.)
+ *
+ * This is nearly the same as heap_deform_tuple(), but for IndexTuples.
+ * One difference is that the tuple should never have any missing columns.
  */
 void
 index_deform_tuple(IndexTuple tup, TupleDesc tupleDescriptor,
                   Datum *values, bool *isnull)
 {
-   int         i;
+   int         hasnulls = IndexTupleHasNulls(tup);
+   int         natts = tupleDescriptor->natts; /* number of atts to extract */
+   int         attnum;
+   char       *tp;             /* ptr to tuple data */
+   int         off;            /* offset in tuple data */
+   bits8      *bp;             /* ptr to null bitmap in tuple */
+   bool        slow = false;   /* can we use/set attcacheoff? */
 
    /* Assert to protect callers who allocate fixed-size arrays */
-   Assert(tupleDescriptor->natts <= INDEX_MAX_KEYS);
+   Assert(natts <= INDEX_MAX_KEYS);
+
+   /* XXX "knows" t_bits are just after fixed tuple header! */
+   bp = (bits8 *) ((char *) tup + sizeof(IndexTupleData));
 
-   for (i = 0; i < tupleDescriptor->natts; i++)
+   tp = (char *) tup + IndexInfoFindDataOffset(tup->t_info);
+   off = 0;
+
+   for (attnum = 0; attnum < natts; attnum++)
    {
-       values[i] = index_getattr(tup, i + 1, tupleDescriptor, &isnull[i]);
+       Form_pg_attribute thisatt = TupleDescAttr(tupleDescriptor, attnum);
+
+       if (hasnulls && att_isnull(attnum, bp))
+       {
+           values[attnum] = (Datum) 0;
+           isnull[attnum] = true;
+           slow = true;        /* can't use attcacheoff anymore */
+           continue;
+       }
+
+       isnull[attnum] = false;
+
+       if (!slow && thisatt->attcacheoff >= 0)
+           off = thisatt->attcacheoff;
+       else if (thisatt->attlen == -1)
+       {
+           /*
+            * We can only cache the offset for a varlena attribute if the
+            * offset is already suitably aligned, so that there would be no
+            * pad bytes in any case: then the offset will be valid for either
+            * an aligned or unaligned value.
+            */
+           if (!slow &&
+               off == att_align_nominal(off, thisatt->attalign))
+               thisatt->attcacheoff = off;
+           else
+           {
+               off = att_align_pointer(off, thisatt->attalign, -1,
+                                       tp + off);
+               slow = true;
+           }
+       }
+       else
+       {
+           /* not varlena, so safe to use att_align_nominal */
+           off = att_align_nominal(off, thisatt->attalign);
+
+           if (!slow)
+               thisatt->attcacheoff = off;
+       }
+
+       values[attnum] = fetchatt(thisatt, tp + off);
+
+       off = att_addlength_pointer(off, thisatt->attlen, tp + off);
+
+       if (thisatt->attlen <= 0)
+           slow = true;        /* can't use attcacheoff anymore */
    }
 }
 

diff --git a/src/backend/executor/nodeIndexonlyscan.c b/src/backend/executor/nodeIndexonlyscan.c
index 72c04b528f5203cc280211daefebaabf7c34b11b..26758e770390d514f007525dd2811865845ea8c6 100644 (file)
--- a/src/backend/executor/nodeIndexonlyscan.c
+++ b/src/backend/executor/nodeIndexonlyscan.c
@@ -269,23 +269,17 @@ IndexOnlyNext(IndexOnlyScanState *node)
 static void
 StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup, TupleDesc itupdesc)
 {
-   int         nindexatts = itupdesc->natts;
-   Datum      *values = slot->tts_values;
-   bool       *isnull = slot->tts_isnull;
-   int         i;
-
    /*
-    * Note: we must use the tupdesc supplied by the AM in index_getattr, not
-    * the slot's tupdesc, in case the latter has different datatypes (this
-    * happens for btree name_ops in particular).  They'd better have the same
-    * number of columns though, as well as being datatype-compatible which is
-    * something we can't so easily check.
+    * Note: we must use the tupdesc supplied by the AM in index_deform_tuple,
+    * not the slot's tupdesc, in case the latter has different datatypes
+    * (this happens for btree name_ops in particular).  They'd better have
+    * the same number of columns though, as well as being datatype-compatible
+    * which is something we can't so easily check.
     */
-   Assert(slot->tts_tupleDescriptor->natts == nindexatts);
+   Assert(slot->tts_tupleDescriptor->natts == itupdesc->natts);
 
    ExecClearTuple(slot);
-   for (i = 0; i < nindexatts; i++)
-       values[i] = index_getattr(itup, i + 1, itupdesc, &isnull[i]);
+   index_deform_tuple(itup, itupdesc, slot->tts_values, slot->tts_isnull);
    ExecStoreVirtualTuple(slot);
 }