EXPLAIN：顯示查詢語句的執行計劃，但不運行。 <br/> explain（執行計劃）基本語法： ```sql explain [extended | dependency | authorization] query ``` ```txt extended：提供執行計劃關于操作的額外信息，比如文件路徑 dependency：提供json格式輸出，包括查詢所依賴的表和分區列表 authorization：列出所有需要授權的實體，包括查詢的輸入輸出和授權失敗 ``` **查看語句的執行計劃** ```sql hive(default)> explain select * from emp; ``` **查看詳細執行計劃** ```sql hive(default)> explain extended select * from emp; ``` 一般顯示如下信息： ```sql STAGE PLANS: Stage: Stage-1 Map Reduce Map Operator Tree: //發生在job的 map 處理階段過程 TableScan　　　　　　　 //讀取表的數據 alias: emp //emp,若select * from emp e;表有別名則顯示別名e Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE Select Operator expressions: age (type: int), name (type: string) outputColumnNames: _col0, _col1 Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE Reduce Output Operator key expressions: _col0 (type: int) sort order: + Map-reduce partition columns: _col0 (type: int) Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE value expressions: _col0 (type: int), _col1 (type: string) Reduce Operator Tree: //發生在job的 reduce 處理階段過程 Extract Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE File Output Operator compressed: false Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE table: //說明輸入格式是TextInputFormat input format: org.apache.hadoop.mapred.TextInputFormat //說明輸出格式是HiveIgnoreKeyTextOutputFormat output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe Stage: Stage-0 Fetch Operator limit: -1 //--job中沒有Limit ``` <br/> **通過工具生成可視化執行計劃**