how to set fetch-size for a query in clojure jdbc?

Question

In the clojure jdbc file of https://github.com/clojure/java.jdbc/blob/master/src/main/clojure/clojure/java/jdbc.clj we could set fetch size for function of prepare-statement. But it's very often to just query a large table and also need to set the fetch size. Won't it better to also provide an option to the function of "query"?

Answer 1

The query function used to allow options to be passed through to the prepare-statement call as part of the "SQL & params" vector but it was not well-documented:

(query [{:fetch-size 50} "SELECT * FROM students WHERE age = ?" 24])

As of java.jdbc 0.6.0-rc2, prepare-statement options can be passed in the normal options map position:

(query ["SELECT * FROM students WHERE age = ?" 24] {:fetch-size 50})

The older way, above, will work on versions prior to 0.6.0-rc2 but is no longer supported (it will throw an exception). See http://dev.clojure.org/jira/browse/JDBC-125 for more details.

Answer 2

If I understand you correctly, you would like to set the maximum number of rows that are returned from a query. In that case, you can use LIMIT , like this

(clojure.java.jdbc/query
 ["SELECT * FROM students WHERE age = 24 ORDER BY created LIMIT 50"])

This should be helpful: Clojure and Large Result Sets