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The following page has been changed by ZhengShao: http://wiki.apache.org/hadoop/Hive/HiveQL/Transform ------------------------------------------------------------------------------ == Transform/Map-Reduce Syntax == + Users can also plug in their own custom mappers and reducers in the data stream by using features natively supported in the Hive 2.0 language. e.g. in order to run a custom mapper script - map_script - and a custom reducer script - reduce_script - the user can issue the following command which uses the TRANSFORM clause to embed the mapper and the reducer scripts. + + Note that columns will be transformed to string and deliminated by TAB before feeding to the user script, and the standard output of the user script will be treated as TAB-separated string columns. User scripts can output debug information to standard error which will be shown on the task detail page on hadoop. + + {{{ - clusterBy: CLUSTER BY colName (, colName)* distributeBy: DISTRIBUTE BY colName (, colName)* sortBy: SORT BY colName (, colName)* @@ -14, +18 @@ query: FROM ( FROM src - MAP ( expression (, expression)* ) + MAP expression (, expression)* USING 'my_map_script' - ( AS (colName (, colName)* ) )? + ( AS colName (, colName)* )? ( clusterBy? | distributeBy? sortBy? ) ) - REDUCE ( expression (, expression)* ) + REDUCE expression (, expression)* USING 'my_reduce_script' - ( AS (colName (, colName)* ) )? + ( AS colName (, colName)* )? }}} + Both ''MAP'' and ''REDUCE'' can be also written as ''SELECT TRANSFORM''. There are actually no difference between these three. + Hive runs the reduce script in the reduce task because of the ''clusterBy''/''distributeBy''/''sortBy'' clause. + + ''clusterBy'' is a short-cut for both ''distributeBy'' and ''sortBy''. + + Hive uses the columns in ''distributeBy'' to distribute the rows among reducers. All rows with the same ''distributeBy'' columns will go to the same reducer. + + Hive uses the columns in ''sortBy'' to sort the rows before feeding the rows to a single reducer. The sort order will be dependent on the column types. If the column is of numeric type, then the sort order is also in numeric order. If the column is of string type, then the sort order will be lexicographical order. + + == Transform/Map-Reduce Examples ==
