Prepared Statements

In Postgres, prepared statements are queries that have passed all the preliminary stages and now ready to be executed. Running the same prepared statement with different parameters is faster than executing a fresh query each time. To prepare a statement, pass a SQL expression into the prepare function. It will return a special PreparedStatement object:

(def stmt-by-id
  (pg/prepare conn "select * from test1 where id = $1"))

(str stmt-by-id)
<Prepared statement, name: s11, param(s): 1, OIDs: [INT4], SQL: select * from test1 where id = $1>

The statement might have parameters. Now that you have a statement, execute it with the execute-statement function. Below, we execute it three times with various primary keys. We also pass the :first? option set to true to have only row in the result.

(pg/execute-statement conn
                      stmt-by-id
                      {:params [1] :first? true})
;; {:name "Ivan", :id 1}

(pg/execute-statement conn
                      stmt-by-id
                      {:params [5] :first? true})
;; {:name "Louie", :id 5}

(pg/execute-statement conn
                      stmt-by-id
                      {:params [8] :first? true})
;; {:name "Agent Smith", :id 8}

During its lifetime on the server, a statement consumes some resources. When it's not needed any longer, release it with the close-statement function:

(pg/close-statement conn stmt-by-id)

The following macro helps to auto-close a statement. The first argument is a binding symbol. It will be pointing to a fresh prepared statement during the execution of the body. Afterwards, the statement is closed.

Below, we insert tree rows in the database using the same prepared statement. Pay attention to the doall clause: it evaluates the lazy sequence produced by for. Without doall, you'll get an error from the server saying there is no such a prepared statement.

(pg/with-statement [stmt conn "insert into test1 (name) values ($1) returning *"]
  (doall
   (for [character ["Agent Brown"
                    "Agent Smith"
                    "Agent Jones"]]
     (pg/execute-statement conn stmt {:params [character] :first? true}))))

({:name "Agent Brown", :id 12}
 {:name "Agent Smith", :id 13}
 {:name "Agent Jones", :id 14})

In Postgres, prepared statements are always bound to a certain connection. Don't share a statement opened in a connection A to B and vice versa. Do not share them across different threads.