THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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By default, columns will be transformed to STRING and delimited by TAB before feeding to the user script; similarly, all NULL values will be converted to the literal string \N in order to differentiate NULL values from empty strings. The standard output of the user script will be treated as TAB-separated STRING columns, any cell containing only \N will be re-interpreted as a NULL, and then the resulting STRING column will be cast to the data type specified in the table declaration in the usual way. User scripts can output debug information to standard error which will be shown on the task detail page on hadoop. These defaults can be overridden with ROW FORMAT ....NOTE:
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It is your responsibility to sanitize any STRING columns prior to transformation. If your STRING column contains tabs, an identity transformer will not give you back what you started with! To help with this, see REGEXP_REPLACE and replace the tabs with some other character on their way into the TRANSFORM() call. |
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Formally, MAP ... and REDUCE ... are syntactic transformations of SELECT TRANSFORM ( ... ). In other words, they serve as comments or notes to the reader of the query. BEWARE: Use of these keywords may be dangerous as (e.g.) typing "REDUCE" does not force a reduce phase to occur and typing "MAP" does not force a new map phase! |
Please also see Sort By / Cluster By / Distribute By and Larry Ogrodnek's blog post.
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