Re: update inside function does not use the index

Johannes schrieb am 16.11.2015 um 14:56:
> I have problems with a self written function, which does not use the
> index, which takes very long (500 ms per update).
>
> The pl/pgsql function iterates over a select resultset with a cursor.
> In every loop I execute an update with a where LIKE condition, which
> relates to my current cursor position:
>
> FOR i IN SELECT id, level_ids, path_names||'%' as path_names from x LOOP
> update x set path_ids[i.level] = id where path_names like i.path_names;
> RAISE NOTICE 'path_names : %', i.path_names;
> END LOOP;
>
> Calling the updates outside the function, they are very fast because
> like 'a.b%' uses the index of the path field ( ~ 15 ms ).


Doing row-by-row processing (also referred to as "slow-by-slow")  is usually not a good idea.

I think your statement can be re-written to avoid the loop completely:

    with path_levels as (
      SELECT id,
             level_ids,
             path_names||'%' as path_names
      from x
    )
    update x
      set path_ids[i.level] = id
    from path_levels i
    where x.path_names like i.path_names