Remove n elements from array using start and end index - sql

Question

I have the following table in a Postgres database:
CREATE TABLE test(
id SERIAL NOT NULL,
arr int[] NOT NULL
)
And the array contains about 500k elements.
I would like to know if there is an efficient way to update arr column by removing a set of elements from the array given the start and end index or just the number of "n first elements" to remove.

Answer 1

You can access individual elements or ranges of elements:
If you e.g. want to remove elements 5 to 8, you can do:
select arr[1:4]||arr[9:]
from test;
or as an update:
update test
set arr = arr[1:4]||arr[9:];
To remove the "first n elements", just use the slice after the n+1 element, e.g. to get remove the first 5 elements:
select arr[6:]
from test;
The syntax arr[6:] requires Postgres 9.6 or later, for earlier versions you need
select arr[6:cardinality(arr)]
from test;
cardinality() was introduced in 9.4, if you are using an even older version, you need:
select arr[6:array_lengt(arr,1)]
from test;

Answer 2

You can use slices (see 8.15.3. Accessing Arrays).
create table example
as select array[1,2,3,4,5,6,7,8] arr;
Remove first 3 elements:
select arr[4:8]
from example;
arr
-------------
{4,5,6,7,8}
(1 row)
Remove elements from 4 to 5:
select arr[1:3] || arr[6:8] as arr
from example;
arr
---------------
{1,2,3,6,7,8}
(1 row)
Remove first 5 elements if the length of the array is unknown:
select arr[6:array_length(arr,1)]
from example;
arr
---------
{6,7,8}
(1 row)