A compute stats command computes the table and column stats for a given
table and persists them in the metastore.
The table stats consist of the per-partition and per-table row count.
The column stats are computed on a per-table basis and consist of the
number of distinct values and the number of NULLs per column.
This patch introduces a new 'child query' concept that
compute stats utilizes. Child queries are cancelled
if the parent query is cancelled. A compute stats stmt is
executed by the following query hirarchy:
parent: compute stats query (DDL)
- child: compute table stats query (QUERY)
- child: compute column stats query (QUERY)
The new child query concept is necessary to decouple child query fetches
from parent query fetches, i.e., we could not execute a child query as
part of the original compute stats query, because then a client could
fetch the results we need for updating the Metastore statistics. The
reason why our existing CTAS works without this decoupling
is that its insert 'child query' is not fetchable.
Change-Id: I560533e3cb09bcbbdb3eea7fcf0b460bc6b36dcd
Reviewed-on: http://gerrit.ent.cloudera.com:8080/873
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins
93e5b262c2