Convert Bit String To Array in PostgreSQL

Question

I have a 160 chars bit string and I need to have an integer array that stores the position of the bits that have a value of 1.

Example:

bitstring = '00110101'
array = [3,4,6,8]

Is it possible to do this just with SQL or do I need to define a PL/SQL function or something like that?

Answer 1

It's assuredly possible to write it in SQL. Here's a starting point:

select array(
  select substring(str from i for 1) as bit
  from generate_series(1, length(str)) as i
  where bit = '1'
);

You might want to wrap that in a pl/sql function regardless, though, so to avoid duplicating code all over the place.

---

Working function:

create or replace function get_bit_positions(varbit) returns bit[] as $$
select array(
  select substring($1 from i for 1) as bit
  from generate_series(1, length($1)) as i
  where substring($1 from i for 1) = '1'
);
$$ language sql immutable;

Answer 2

Working version:

WITH x AS (SELECT '00110101'::varbit AS b)
SELECT array_agg(i)
FROM  (SELECT b, generate_series(1, length(b)) AS i FROM x) y
WHERE  substring(b, i, 1) = '1';

Simpler once you convert the varbit to text[]. Cast to text and run string_to_array().

Then you can use generate_subscripts() and pick array elements by index:

WITH x AS (SELECT string_to_array('00110101'::varbit::text, NULL) AS b)
SELECT array_agg(i)
FROM  (SELECT b, generate_subscripts(b,1) AS i FROM x) y
WHERE b[i] = '1'

Details in this related question on dba.SE.