Re: Proposal to introduce a shuffle function to intarray extension

[Martin Kalcher]

Am 18.07.22 um 01:20 schrieb Tom Lane:
(Having said that, even if we were going to implement it with that
definition, I should think that it'd be easiest to do so on the
array-of-Datums representation produced by deconstruct_array.
That way you don't need to do different things for different element
types.)

Thank you Tom, here is a patch utilising deconstruct_array(). If we 
agree, that this is the way to go, i would like to add array_sample() 
(good name?), some test cases, and documentation.

One more question. How do i pick a Oid for the functions?

MartinFrom baec08168357098287342c92672ef97361a91371 Mon Sep 17 00:00:00 2001
From: Martin Kalcher <martin.kalc...@aboutsource.net>
Date: Sun, 17 Jul 2022 18:06:04 +0200
Subject: [PATCH] introduce array_shuffle()

---
 src/backend/utils/adt/arrayfuncs.c | 61 ++++++++++++++++++++++++++++++
 src/include/catalog/pg_proc.dat    |  3 ++
 2 files changed, 64 insertions(+)

diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index fb167f226a..e185c1ef74 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/src/backend/utils/adt/arrayfuncs.c
@@ -6872,3 +6872,64 @@ trim_array(PG_FUNCTION_ARGS)
 
 	PG_RETURN_DATUM(result);
 }
+
+Datum
+array_shuffle(PG_FUNCTION_ARGS)
+{
+	ArrayType		*array,
+					*result;
+	int16			elmlen;
+	bool			elmbyval;
+	char			elmalign;
+	Oid             elmtyp;
+	TypeCacheEntry	*typentry;
+	Datum			*elms,
+					elm;
+	bool			*nuls,
+					nul;
+	int         	nelms,
+					i,
+					j;
+
+	array = PG_GETARG_ARRAYTYPE_P(0);
+
+	if (ArrayGetNItems(ARR_NDIM(array), ARR_DIMS(array)) < 2)
+		PG_RETURN_ARRAYTYPE_P(array);
+
+	elmtyp = ARR_ELEMTYPE(array);
+
+	typentry = (TypeCacheEntry *) fcinfo->flinfo->fn_extra;
+
+	if (typentry == NULL || typentry->type_id != elmtyp)
+	{
+		typentry = lookup_type_cache(elmtyp, 0);
+		fcinfo->flinfo->fn_extra = (void *) typentry;
+	}
+	elmlen = typentry->typlen;
+	elmbyval = typentry->typbyval;
+	elmalign = typentry->typalign;
+
+	deconstruct_array(array, elmtyp, elmlen, elmbyval, elmalign,
+					  &elms, &nuls, &nelms);
+
+	for (i = nelms - 1; i > 0; i--)
+	{
+		j = random() % (i + 1);
+		elm = elms[i];
+		nul = nuls[i];
+		elms[i] = elms[j];
+		nuls[i] = nuls[j];
+		elms[j] = elm;
+		nuls[j] = nul;
+	}
+
+	result = construct_md_array(elms, nuls,
+								ARR_NDIM(array), ARR_DIMS(array), ARR_LBOUND(array),
+								elmtyp, elmlen, elmbyval, elmalign);
+
+	pfree(elms);
+	pfree(nuls);
+	PG_FREE_IF_COPY(array, 0);
+
+	PG_RETURN_ARRAYTYPE_P(result);
+}
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 2e41f4d9e8..56aff551d3 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -1681,6 +1681,9 @@
   proname => 'arraycontjoinsel', provolatile => 's', prorettype => 'float8',
   proargtypes => 'internal oid internal int2 internal',
   prosrc => 'arraycontjoinsel' },
+{ oid => '7777', descr => 'shuffle array',
+  proname => 'array_shuffle', proisstrict => 'f', prorettype => 'anyarray',
+  proargtypes => 'anyarray', prosrc => 'array_shuffle' },
 
 { oid => '764', descr => 'large object import',
   proname => 'lo_import', provolatile => 'v', proparallel => 'u',
-- 
2.37.1