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Editorial Team
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Asked: 2026-06-10T11:05:58+00:00

According to the release notes for the MySQL JDBC driver , it should stream

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According to the release notes for the MySQL JDBC driver, it should stream results if and only if using a connection with concurrency read-only, forward-only results, and a fetch-size of exactly Integer/MIN_VALUE.

However, when I attempt to generate exactly these conditions (against [mysql/mysql-connector-java "5.1.21"]), my SQL query still runs forever (or, rather, until it exhausts the JVM’s memory and goes boom).

(let [query (query-only (fetch-all big-table))]
  (clojure.java.jdbc/with-connection (get-connection (:db query))
    (clojure.java.jdbc/with-query-results rows
      (into [{:fetch-size Integer/MIN_VALUE
              :concurrency :read-only
              :result-type :forward-only} (:sql-str query)]
            (:params query))
      (throw (Exception. (str "retrieved a row: " (pr-str (first rows)))))))))
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1 Answer

  1. Editorial Team

    This answer refers to postgresql instead of MySQL, but should apply to both.

    Wrap your with-query-results function with (clojure.java.jdbc/transaction), so:

    (let [query (query-only (fetch-all big-table))]
  (clojure.java.jdbc/with-connection (get-connection (:db query))
    (clojure.java.jdbc/transaction        
      (clojure.java.jdbc/with-query-results rows
        (into [{:fetch-size Integer/MIN_VALUE
                :concurrency :read-only
                :result-type :forward-only} (:sql-str query)]
              (:params query))
        (throw (Exception. (str "retrieved a row: " (pr-str (first rows))))))))))

    The postgresql docs specify one more requirement for enabling streaming: “The Connection must not be in autocommit mode.” By default the connection is created with autocommit on, but wrapping with (clojure.java.jdbc/transaction) will run the inner code with autocommit off. You could also call .setAutoCommit on the connection yourself.

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