Hello,
The attached patch improves the performance of array_length() by detoasting
only the overhead part of the datum.
Here is a test case:
postgres=# create table array_length_test as select array_agg(a) a from
generate_series(1, 10000) a, generate_series(1, 10000) b group by b;
Without the patch:
postgres=# select sum(array_length(a, 1)) from array_length_test;
sum
-----------
100000000
(1 row)
Time: 199.002 ms
With the patch:
postgres=# select sum(array_length(a, 1)) from array_length_test;
sum
-----------
100000000
(1 row)
Time: 34.599 ms
The motivation for patch is that some of our customers use arrays to store
a sequence of tens of thousands of events in each row. They often need to
get the last 10 event for each row, for which we do A[array_length(A, 1) -
9: 1000000] (assuming 1M is an upper-bound. we could use array_length()
instead of this constant too, but that is unnecessary if we know the
upper-bound and only slows down the query). Without this optimization,
array gets detoasted twice. With this patch, array_length() becomes much
faster, and the whole query saves few seconds.
Of course this technique is applicable to some other functions too, but
they have never become a bottleneck for me, so I decided to keep the
changes only to this function.
Another alternative I could think of was introducing python style slicing,
in which negative indexes start from end of array, so -10 means 10th
element from end. I thought this would be a bigger change and is probably
unnecessary, so I decided to improve array_length() instead.
Feedback is welcome.
Thanks,
-- Hadi
diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index 91df184..5e1d9c2 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/src/backend/utils/adt/arrayfuncs.c
@@ -1719,11 +1719,15 @@ array_upper(PG_FUNCTION_ARGS)
Datum
array_length(PG_FUNCTION_ARGS)
{
- ArrayType *v = PG_GETARG_ARRAYTYPE_P(0);
+ Datum arrdatum = PG_GETARG_DATUM(0);
int reqdim = PG_GETARG_INT32(1);
+ ArrayType *v;
int *dimv;
int result;
+ v = (ArrayType *) PG_DETOAST_DATUM_SLICE(arrdatum, 0,
+ ARR_OVERHEAD_NONULLS(MAXDIM));
+
/* Sanity check: does it look like an array at all? */
if (ARR_NDIM(v) <= 0 || ARR_NDIM(v) > MAXDIM)
PG_RETURN_NULL();
--
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