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From Eugene Koifman <ekoif...@hortonworks.com>
Subject Make DESCRIBE <table> return info from SerDe?
Date Tue, 06 May 2014 01:31:46 GMT
Hi,
is it possible to write a SerDe (or something else) so that DESCRIBE
<table> (as we all query parser) get the column definitions from the SerDe
rather than Metastore?

The idea is to support self-describing data types such as JSON such that
when the per partition schema changes, there is no need to run an ALTER
TABLE to access new fields.

For example, partition 1 of table T has JSON docs with fields 'a', 'b', 'c'
and the table was created with schema that has 3 columns.
Let's say later a partition is added where the docs have fields 'a', 'b',
'c', 'd'.

Is it possible to then run a query 'SELECT T.d from T' w/o ALTER TABLE?
 (assuming SerDe is smart enough to produce NULL for row that don't have
this field)

Is there a mechanism for that?

Thank you,
Eugene

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