clojure.java.jdbc /查询大结果集懒惰

Question

我正在尝试从数据库中读取数百万行并写入文本文件.

这是我的问题数据库转储到带有副作用的文本文件的延续

我现在的问题似乎是在程序完成之前不会发生日志记录.我没有懒惰处理的另一个指标是在程序完成之前根本不写入文本文件.

基于一个IRC尖看来我的问题很可能具有做:result-set-fn,缺省值为doall在clojure.java.jdbc/query代码的区域.

我试图用一个for函数替换它,但仍然发现内存消耗很高,因为它将整个结果集拉入内存.

我怎么能有一个:result-set-fn不会把所有东西都拉进来的doall？如何在程序运行时逐步编写日志文件,而不是在-main执行完成后转储所有内容？

    (let [ 
          db-spec              local-postgres
          sql                  "select * from public.f_5500_sf "
          log-report-interval  1000
          fetch-size           100
          field-delim          "\t"                                                                  
          row-delim            "\n"                                                                  
          db-connection        (doto ( j/get-connection db-spec) (.setAutoCommit false)) 
          statement            (j/prepare-statement db-connection sql :fetch-size fetch-size ) 
          joiner               (fn [v] (str (join field-delim v ) row-delim ) )                      
          start                (System/currentTimeMillis)                                            
          rate-calc            (fn [r] (float (/ r (/ ( - (System/currentTimeMillis) start) 100))))  
          row-count            (atom 0)                                                              
          result-set-fn        (fn [rs] (lazy-seq rs))
          lazy-results         (rest (j/query db-connection [statement] :as-arrays? true :row-fn joiner :result-set-fn result-set-fn)) 
          ]; }}}
      (.setAutoCommit db-connection false)
      (info "Started dbdump session...")    
      (with-open [^java.io.Writer wrtr (io/writer "output.txt")]
        (info "Running query...")    
        (doseq [row lazy-results] 
          (.write wrtr row)
          ))  
        (info (format "Completed write with %d rows"   @row-count))
      )

Answer 1

我clojure.java.jdbc通过放入[org.clojure/java.jdbc "0.3.0-beta1"]我的project.clj依赖项列表来获取最近的修复.这个增强/纠正了这里描述的:as-arrays? true功能.clojure.java.jdbc/query

我认为这有点帮助,但我仍然能够覆盖:result-set-fn到vec.

核心问题通过将所有行逻辑塞进来解决:row-fn.最初的OutOfMemory问题与迭代j/query结果集而不是定义特定问题有关:row-fn.

新(工作)代码如下:

(defn -main []
  (let [; {{{
        db-spec              local-postgres
        source-sql           "select * from public.f_5500 "
        log-report-interval  1000
        fetch-size           1000
        row-count            (atom 0)
        field-delim          "\u0001"   ; unlikely to be in source feed,
                                        ; although i should still check in
                                        ; replace-newline below (for when "\t"
                                        ; is used especially) 
        row-delim            "\n" ; unless fixed-width, target doesn't
                                  ; support non-printable chars for recDelim like 
        db-connection        (doto ( j/get-connection db-spec) (.setAutoCommit false))
        statement            (j/prepare-statement db-connection source-sql :fetch-size fetch-size :concurrency :read-only)
        start                (System/currentTimeMillis)
        rate-calc            (fn [r] (float (/ r (/ ( - (System/currentTimeMillis) start) 100))))
        replace-newline      (fn [s] (if (string? s) (clojure.string/replace  s #"\n" " ") s))
        row-fn               (fn [v] 
                               (swap! row-count inc)
                               (when (zero? (mod @row-count log-report-interval))
                                 (info (format "wrote %d rows" @row-count))
                                 (info (format "\trows/s %.2f"  (rate-calc @row-count)))
                                 (info (format "\tPercent Mem used %s "  (memory-percent-used))))
                               (str (join field-delim (doall (map #(replace-newline %) v))) row-delim ))
        ]; }}}
    (info "Started database table dump session...")
    (with-open [^java.io.Writer wrtr (io/writer "./sql/output.txt")]
      (j/query db-connection [statement] :as-arrays? true :row-fn 
               #(.write wrtr (row-fn %))))
    (info (format "\t\t\tCompleted with %d rows" @row-count))
    (info (format "\t\t\tCompleted in %s seconds" (float (/ (- (System/currentTimeMillis) start) 1000))))
    (info (format "\t\t\tAverage rows/s %.2f"  (rate-calc @row-count)))
    nil)
  )

我试验的其他事情(成功有限)涉及音色记录和关闭标准; 我想知道如果使用REPL它可能会在显示回我的编辑器(vim壁炉)之前缓存结果,我不确定这是否利用了大量的内存.

另外,我在内存中添加了记录部分 (.freeMemory (java.lang.Runtime/getRuntime)).我对VisualVM并不熟悉并准确指出我的问题所在.

我很高兴现在的工作方式,感谢大家的帮助.