Converting function from Oracle to PostgreSQL

Question

I am working on converting something from Oracle to PostgreSQL. In the Oracle file there is a function:

instr(string,substring,starting point,nth location)

or as it is in my file

instr(string,chr(10),instr(string,substring),1)

In PostgreSQL this does not exist, so I looked up an equivalent function. I found:

position(substring in string)

but this does not allow the starting position and the nth location parameters.

Is there anyway to make this function start at a given point? Or is there a better function to use in PostgreSQL where I can specify starting position and the nth location?

This would have to work on PostgreSQL 8.2.15 because that is the version we are running on the database.

Answer 1

The function strpos(str, sub) in Postgres is equivalent of instr(str, sub) in Oracle. Unfortunately, the function does not have third and fourth parameters, so the expression in Postgres must be more complex.

The function substr(str, n) gives a substring of str starting from n position.

instr(str, ch, instr(str, sub), 1);                               --oracle
strpos(substr(str, strpos(str, sub)), ch) + strpos(str, sub) - 1; --postgres

As instr() is a powerful function I wrote it in plpgsql for my own needs.

create or replace function instr(str text, sub text, startpos int = 1, occurrence int = 1)
returns int language plpgsql immutable
as $$
declare 
    tail text;
    shift int;
    pos int;
    i int;
begin
    shift:= 0;
    if startpos = 0 or occurrence <= 0 then
        return 0;
    end if;
    if startpos < 0 then
        str:= reverse(str);
        sub:= reverse(sub);
        pos:= -startpos;
    else
        pos:= startpos;
    end if;
    for i in 1..occurrence loop
        shift:= shift+ pos;
        tail:= substr(str, shift);
        pos:= strpos(tail, sub);
        if pos = 0 then
            return 0;
        end if;
    end loop;
    if startpos > 0 then
        return pos+ shift- 1;
    else
        return length(str)- length(sub)- pos- shift+ 3;
    end if;
end $$;

Some checks (Examples from OLAP DML Functions):

select instr('Corporate Floor', 'or', 3, 2);  -- gives 14
select instr('Corporate Floor', 'or', -3, 2); -- gives 2

There is no reverse() function in Postgres 8.2. You can use this:

-- only for Postgres 8.4 or earlier!
create or replace function reverse(str text)
returns text language plpgsql immutable
as $$
declare
    i int;
    res text = '';
begin
    for i in 1..length(str) loop
        res:= substr(str, i, 1) || res;
    end loop;
    return res;
end $$;

Note. Postgres 15 introduced the regexp_instr() function.

regexp_instr ( string text, pattern text [, start integer [, N integer [, endoption integer [, flags text [, subexpr integer ] ] ] ] ] ) → integer