postgres.clj

(ns queries.postgres
  (:gen-class)
  (:require [cascalog.ops :as c])
  (:use [cascalog.api])
  (:import [com.twitter.maple.jdbc JDBCTap JDBCScheme TableDesc]
           [cascading.tap.SinkMode]))

(defn db-tap [table]
  (let [tap (com.twitter.maple.jdbc.JDBCTap.
      "jdbc:postgresql://my-db-uri?ssl=true&sslfactory=org.postgresql.ssl.NonValidatingFactory"
      "username"
      "password"
      "org.postgresql.Driver"
      (com.twitter.maple.jdbc.TableDesc. table (into-array ["id" "timestamp"]) (into-array ["bigserial" "timestamp"]) (into-array ["id"]))
      (com.twitter.maple.jdbc.JDBCScheme. 
        (into-array ["id" "timestamp"])
        (str "select id, timestamp from " table " order by id DESC")
        (str "select id from " table " order by id DESC limit 1"
        10000) ;; SET LIMIT
      (cascading.tap.SinkMode/UPDATE)))]
      (.setConcurrentReads tap 100) ;; means 100 of limit 100 (limit / concurrent reads)
      tap))

(defn -main [& args]
  (?<- 
    (hfs-textline "output.txt")
       [?id ?timestamp]
       ((db-tap "my_table") ?id ?timestamp)
       (c/count ?count))

project.clj

(defproject my-analysis "0.1.0-SNAPSHOT"
  :description "TODO: Project Description."  
  :source-paths ["src/clj"]
  :jvm-opts ["-Xmx768m" "-server"]

  :repositories {"conjars" "http://conjars.org/repo"}

  :dependencies [ 
                  [org.clojure/clojure "1.3.0"]
                  [cascalog "1.9.0"]
                  
                  ;; Cascalog JDBC tap
                  [com.twitter/maple "0.2.2"]
                  
                  ;; JDBC driver for postgres
                  [postgresql/postgresql "9.1-901.jdbc4"]

                  ;; Command line args processing
                  [org.clojure/tools.cli "0.2.1"]
                ]
  :profiles { 
    :dev { 
      :dependencies [
                      [org.apache.hadoop/hadoop-core "0.20.2-dev"]
                      [midje-cascalog "0.4.0"]
                      [lein-midje "1.0.10"]
                     ]}})