Given a string column with a value similar to /123/12/34/56/5/, what is the optimal way of querying for all the records that include the given number (12 for example)?
The solution from top of my head is:
SELECT id FROM things WHERE things.path LIKE '%/12/%'
But AFAIK this query can't use indexes on the column due to the leading %.
There must be something better. What is it?
Using PostgreSQL, but would prefer the solution that would work across other DBs too.
In PostgreSQL 9.1 you could utilize the pg_trgm module and build a GIN index with it.
CREATE EXTENSION pg_trgm; -- once per database
CREATE INDEX things_path_trgm_gin_idx ON things USING gin (path gin_trgm_ops);
Your LIKE expression can use this index even if it is not left-anchored.
See a detailed demo by depesz here.
Normalize it If you can, though.
If you're happy turning that column into an array of integers, like:
'/123/12/34/56/5/' becomes ARRAY[123,12,34,56,5]
So that path_arr is a column of type INTEGER[], then you can create a GIN index on that column:
CREATE INDEX ON things USING gin(path_arr);
A query for all items containing 12 then becomes:
SELECT * FROM things WHERE ARRAY[12] <@ path_arr;
Which will use the index. In my test (with a million rows), I get plans like:
EXPLAIN SELECT * FROM things WHERE ARRAY[12] <@ path_arr;
QUERY PLAN
----------------------------------------------------------------------------------------
Bitmap Heap Scan on things (cost=5915.75..9216.99 rows=1000 width=92)
Recheck Cond: (path_arr <@ '{12}'::integer[])
-> Bitmap Index Scan on things_path_arr_idx (cost=0.00..5915.50 rows=1000 width=0)
Index Cond: ('{12}'::integer[] <@ path_arr)
(4 rows)
