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https://issues.apache.org/jira/browse/MAPREDUCE-1224?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780349#action_12780349
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Todd Lipcon commented on MAPREDUCE-1224:
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Perhaps this could be changed to add "WHERE 1 = 0". Any SQL optimizer should
evaluate this very quickly and return an empty result set, allowing metadata to
be grabbed without actually doing work. Aaron?
> Calling "SELECT t.* from <table> AS t" to get meta information is too
> expensive for big tables
> ----------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-1224
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1224
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: contrib/sqoop
> Affects Versions: 0.20.1
> Environment: all platforms, generic jdbc driver
> Reporter: Spencer Ho
>
> The SqlManager uses the query, "SELECT t.* from <table> AS t" to get table
> spec is too expensive for big tables, and it was called twice to generate
> column names and types. For tables that are big enough to be map-reduced,
> this is too expensive to make sqoop useful.
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