What is the best way to run a longest matching prefix against a table column?

Question

I need to run a longest matching prefix against a column in a table, not just a single value. For a single value I use something like SELECT value, prefix as lmp FROM aTable WHERE SUBSTRING(value,1, LENGTH(prefix)) = prefix ORDER BY prefix DESC limit 1.

The problem is if it is being done against many records it will entail doing a table scan and getting the values one by one, and will entail a lot of traffic between client and server.

Is there a way to do it in a single query which will involve subqueries but not stored procedures? I am using PostgreSQL 8.4.

score 1 · Accepted Answer · answered Mar 05 '13 at 16:03

You can put the values in a table using a CTE and do:

with list as (
    select 'abc' as prefix union all
    . . .
)
select value, lmp
from (SELECT value, prefix as lmp,
             row_number() over (partition by value order by len(prefix) desc) as seqnum
      FROM aTable join
           list l
           on SUBSTRING(value,1, LENGTH(prefix)) = prefix
     ) t
where seqnum = 1

This will move the looping entirely onto the server, which should be a bit faster.

What is the best way to run a longest matching prefix against a table column?

1 Answers1