Question

PostgreSQL 7.4 (Yep upgrading)

So in my WHERE condition I have this

AND CASE
    WHEN "substring"(t."FieldID"::text, 0, 3) = '01'::text 
        OR "substring"(t."FieldID"::text, 0, 4) = '123'::text 
        OR "substring"(t."FieldID"::text, 0, 5) = '5555'::text 
        OR "substring"(t."FieldID"::text, 0, 6) = '44444'::text 
        OR "substring"(t."FieldID"::text, 0, 3) = '99'::text 
    THEN 1
    ELSE 0
END = 1

Alternate syntax but no change in Cost

AND CASE
    WHEN "substring"(t."FieldID"::text, 0, 3) = '01'::text THEN 1
    WHEN "substring"(t."FieldID"::text, 0, 4) = '123'::text THEN 1
    WHEN "substring"(t."FieldID"::text, 0, 5) = '5555'::text THEN 1
    WHEN "substring"(t."FieldID"::text, 0, 6) = '44444'::text THEN 1
    WHEN "substring"(t."FieldID"::text, 0, 3) = '99'::text THEN 1    
    ELSE 0
END = 1

Looking for a Cost effective way to limit the results by the start of a string. So if the string starts with 01, 123, 5555, 44444 or 99 add it to the result set.

Any thoughts?

Note: the FieldID is indexed
Viewing the Explain data to see the bottle necks in the query, when adding the above code is when the cost of the Sort goes way up and slows the return of the data set/results.

Output from Explain:

Sort (cost=88716.84..88719.89 rows=822 width=64)

there is a ton more as the query is complex but if I remove the part of the code the Sort cost goes way down

Answer 1

If you are just filtering by the starting chars, you can use like with no problem and it will just use an index.

AND (t."FieldID"::text LIKE '01%' OR 
     t."FieldID"::text LIKE '123%' OR 
     t."FieldID"::text LIKE '5555%' OR
     t."FieldID"::text LIKE '44444%' OR
     t."FieldID"::text LIKE '99%')