Postgres-XC tutorial PDF - PGCon

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Distributed tables – multi­row operation (2)
Example: aggregation on distributed table
EXPLAIN VERBOSE SELECT sum(val) FROM tab1 GROUP BY val2;
QUERY PLAN
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HashAggregate
Output: pg_catalog.sum((sum(tab1.val))), tab1.val2
­­ Finalise the grouping and aggregation at coordinator
­> Data Node Scan on "__REMOTE_GROUP_QUERY__"
Output: sum(tab1.val), tab1.val2
Node/s: datanode_1, datanode_2, datanode_3
Remote query: SELECT sum(group_1.val), group_1.val2
FROM (SELECT val, val2 FROM ONLY tab1
WHERE true) group_1 GROUP BY 2
­­ Get partially grouped and aggregated results from datanodes
May 16th, 2012 Postgres­XC 54