storing varying data types in a (postgresql) database
CREATE TABLE key_values(user_id integer references users(id), key varchar(255), type varchar(255), CONSTRAINT UNIQUE(user_id, key));
CREATE TABLE key_values_int(value integer) INHERITS (key_values);
CREATE TABLE key_values_string(value varchar(255)) INHERITS (key_values);
and so on...

CREATE INDEX tbl_col_propx_idx ON tbl (cast(col AS int))
WHERE  prop_type_id = 23;  -- 23 signifying "propertyX"

CREATE TABLE prop_type(
   prop_type_id int PRIMARY KEY,
   prop_type text NOT NULL);

SELECT *
FROM   tbl
WHERE  prop_type_id = 23   -- 23 signifying "propertyX"
AND    col::int BETWEEN 50 AND 100