On 3/2/21 1:02 AM, Dian M Fay wrote:
> On Mon Mar 1, 2021 at 6:53 PM EST, Vik Fearing wrote:
>>> This basically does what it says, and the code looks good. The
>>> documentation is out of alphabetical order (trim_array should appear
>>> under cardinality rather than over)) but good otherwise.
>>
>> Hmm. It appears between cardinality and unnest in the source code and
>> also my compiled html. Can you say more about where you're seeing the
>> wrong order?
>
> I applied the patch to the latest commit, ffd3944ab9. Table 9.52 is
> ordered:
>
> array_to_string
> array_upper
> trim_array
> cardinality
> unnest
So it turns out I must have fixed it locally after I posted the patch
and then forgot I did that. Attached is a new patch with the order
correct. Thanks for spotting it!
>> The problem here is that postgres needs to know what the return
>> type is and it can only determine that from the input.
>>
>> If you give the function a typed null, it returns null as expected.
>>
>>> The new status of this patch is: Waiting on Author
>>
>> I put it back to Needs Review without a new patch because I don't know
>> what I would change.
>
> I'd thought that checking v and returning null instead of raising the
> error would be more friendly, should it be possible to pass an untyped
> null accidentally instead of on purpose, and I couldn't rule that out.
As Tom said, that is something that does not belong in this patch.
--
Vik Fearing
>From 351bf14400d8ea2948788a96b47ce6c20847de97 Mon Sep 17 00:00:00 2001
From: Vik Fearing <v...@postgresfriends.org>
Date: Tue, 16 Feb 2021 18:38:24 +0100
Subject: [PATCH] implement trim_array
---
doc/src/sgml/func.sgml | 18 ++++++++++++
src/backend/catalog/sql_features.txt | 2 +-
src/backend/utils/adt/arrayfuncs.c | 42 ++++++++++++++++++++++++++++
src/include/catalog/pg_proc.dat | 5 ++++
src/test/regress/expected/arrays.out | 23 +++++++++++++++
src/test/regress/sql/arrays.sql | 19 +++++++++++++
6 files changed, 108 insertions(+), 1 deletion(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 08f08322ca..2aac87cf8d 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -17909,6 +17909,24 @@ SELECT NULLIF(value, '(none)') ...
</para></entry>
</row>
+ <row>
+ <entry role="func_table_entry"><para role="func_signature">
+ <indexterm>
+ <primary>trim_array</primary>
+ </indexterm>
+ <function>trim_array</function> ( <parameter>array</parameter> <type>anyarray</type>, <parameter>n</parameter> <type>integer</type> )
+ <returnvalue>anyarray</returnvalue>
+ </para>
+ <para>
+ Trims an array by removing the last <parameter>n</parameter> elements.
+ If the array is multidimensional, only the first dimension is trimmed.
+ </para>
+ <para>
+ <literal>trim_array(ARRAY[1,2,3,4,5,6], 2)</literal>
+ <returnvalue>{1,2,3,4}</returnvalue>
+ </para></entry>
+ </row>
+
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index ab0895ce3c..32eed988ab 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -398,7 +398,7 @@ S301 Enhanced UNNEST YES
S401 Distinct types based on array types NO
S402 Distinct types based on distinct types NO
S403 ARRAY_MAX_CARDINALITY NO
-S404 TRIM_ARRAY NO
+S404 TRIM_ARRAY YES
T011 Timestamp in Information Schema NO
T021 BINARY and VARBINARY data types NO
T022 Advanced support for BINARY and VARBINARY data types NO
diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index f7012cc5d9..d38a99f0b0 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/src/backend/utils/adt/arrayfuncs.c
@@ -6631,3 +6631,45 @@ width_bucket_array_variable(Datum operand,
return left;
}
+
+/*
+ * Trim the right N elements from an array by calculating an appropriate slice.
+ * Only the first dimension is trimmed.
+ */
+Datum
+trim_array(PG_FUNCTION_ARGS)
+{
+ ArrayType *v = PG_GETARG_ARRAYTYPE_P(0);
+ int n = PG_GETARG_INT32(1);
+ int array_length = ARR_DIMS(v)[0];
+ int16 elmlen;
+ bool elmbyval;
+ char elmalign;
+ int lower[MAXDIM];
+ int upper[MAXDIM];
+ bool lowerProvided[MAXDIM];
+ bool upperProvided[MAXDIM];
+ Datum result;
+
+ /* Throw an error if out of bounds */
+ if (n < 0 || n > array_length)
+ ereport(ERROR,
+ (errcode(ERRCODE_ARRAY_ELEMENT_ERROR),
+ errmsg("number of elements to trim must be between 0 and %d", array_length)));
+
+ /* Set all the bounds as unprovided except the first upper bound */
+ memset(lowerProvided, 0, sizeof(lowerProvided));
+ memset(upperProvided, 0, sizeof(upperProvided));
+ upper[0] = ARR_LBOUND(v)[0] + array_length - n - 1;
+ upperProvided[0] = true;
+
+ /* Fetch the needed information about the element type */
+ get_typlenbyvalalign(ARR_ELEMTYPE(v), &elmlen, &elmbyval, &elmalign);
+
+ /* Get the slice */
+ result = array_get_slice(PointerGetDatum(v), 1,
+ upper, lower, upperProvided, lowerProvided,
+ -1, elmlen, elmbyval, elmalign);
+
+ PG_RETURN_DATUM(result);
+}
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 1487710d59..8ab911238d 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -1674,6 +1674,11 @@
proname => 'arraycontjoinsel', provolatile => 's', prorettype => 'float8',
proargtypes => 'internal oid internal int2 internal',
prosrc => 'arraycontjoinsel' },
+{ oid => '8819',
+ descr => 'trim an array down to n elements',
+ proname => 'trim_array', proisstrict => 't', provolatile => 'i',
+ prorettype => 'anyarray', proargtypes => 'anyarray int4',
+ prosrc => 'trim_array' },
{ oid => '764', descr => 'large object import',
proname => 'lo_import', provolatile => 'v', proparallel => 'u',
diff --git a/src/test/regress/expected/arrays.out b/src/test/regress/expected/arrays.out
index 8bc7721e7d..fd3e4bfc49 100644
--- a/src/test/regress/expected/arrays.out
+++ b/src/test/regress/expected/arrays.out
@@ -2399,3 +2399,26 @@ SELECT width_bucket(5, ARRAY[3, 4, NULL]);
ERROR: thresholds array must not contain NULLs
SELECT width_bucket(5, ARRAY[ARRAY[1, 2], ARRAY[3, 4]]);
ERROR: thresholds must be one-dimensional array
+-- trim_array
+CREATE TABLE trim_array_test (arr integer[]);
+INSERT INTO trim_array_test
+VALUES ('[-15:-10]={1,2,3,4,5,6}'),
+ ('{1,2,3,4,5,6}'),
+ ('[10:15]={1,2,3,4,5,6}'),
+ ('{{1,10},{2,20},{3,30},{4,40},{5,50},{6,60}}');
+SELECT arr, trim_array(arr, 2)
+FROM trim_array_test
+ORDER BY arr;
+ arr | trim_array
+---------------------------------------------+-------------------------------
+ [-15:-10]={1,2,3,4,5,6} | {1,2,3,4}
+ {1,2,3,4,5,6} | {1,2,3,4}
+ [10:15]={1,2,3,4,5,6} | {1,2,3,4}
+ {{1,10},{2,20},{3,30},{4,40},{5,50},{6,60}} | {{1,10},{2,20},{3,30},{4,40}}
+(4 rows)
+
+DROP TABLE trim_array_test;
+VALUES (trim_array(ARRAY[1, 2, 3], -1)); -- fail
+ERROR: number of elements to trim must be between 0 and 3
+VALUES (trim_array(ARRAY[1, 2, 3], 10)); -- fail
+ERROR: number of elements to trim must be between 0 and 3
diff --git a/src/test/regress/sql/arrays.sql b/src/test/regress/sql/arrays.sql
index c40619a8d5..551cf5c5c9 100644
--- a/src/test/regress/sql/arrays.sql
+++ b/src/test/regress/sql/arrays.sql
@@ -722,3 +722,22 @@ SELECT width_bucket(5, '{}');
SELECT width_bucket('5'::text, ARRAY[3, 4]::integer[]);
SELECT width_bucket(5, ARRAY[3, 4, NULL]);
SELECT width_bucket(5, ARRAY[ARRAY[1, 2], ARRAY[3, 4]]);
+
+
+-- trim_array
+
+CREATE TABLE trim_array_test (arr integer[]);
+INSERT INTO trim_array_test
+VALUES ('[-15:-10]={1,2,3,4,5,6}'),
+ ('{1,2,3,4,5,6}'),
+ ('[10:15]={1,2,3,4,5,6}'),
+ ('{{1,10},{2,20},{3,30},{4,40},{5,50},{6,60}}');
+
+SELECT arr, trim_array(arr, 2)
+FROM trim_array_test
+ORDER BY arr;
+
+DROP TABLE trim_array_test;
+
+VALUES (trim_array(ARRAY[1, 2, 3], -1)); -- fail
+VALUES (trim_array(ARRAY[1, 2, 3], 10)); -- fail
--
2.25.1
