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From "Gerald Sangudi (JIRA)" <j...@apache.org>
Subject [jira] [Created] (PHOENIX-4751) Support hash aggregation with SORT_MERGE_JOIN
Date Wed, 23 May 2018 17:42:00 GMT
Gerald Sangudi created PHOENIX-4751:
---------------------------------------

             Summary: Support hash aggregation with SORT_MERGE_JOIN
                 Key: PHOENIX-4751
                 URL: https://issues.apache.org/jira/browse/PHOENIX-4751
             Project: Phoenix
          Issue Type: Improvement
    Affects Versions: 4.14.0, 4.13.1
            Reporter: Gerald Sangudi


A GROUP BY that follows a SORT_MERGE_JOIN should be able to use hash aggregation in some
cases, for improved performance.

When a GROUP BY follows a SORT_MERGE_JOIN, the GROUP BY does not use hash aggregation. It
instead performs a CLIENT SORT followed by a CLIENT AGGREGATE. The performance can be improved
if (a) the GROUP BY output does not need to be sorted, and (b) the GROUP BY input is large
enough and has low cardinality.

The hash aggregation can initially be a hint. Here is an example from Phoenix 4.13.1 that
would benefit from hash aggregation if the GROUP BY input is large with low cardinality.

{{CREATE TABLE unsalted (}}
{{ keyA BIGINT NOT NULL,}}
{{ keyB BIGINT NOT NULL,}}
{{ val SMALLINT,}}
{{ CONSTRAINT pk PRIMARY KEY (keyA, keyB)}}
{{ );}}

{{EXPLAIN}}
{{ SELECT /*+ USE_SORT_MERGE_JOIN */ }}
{{ t1.val v1, t2.val v2, COUNT(*) c }}
{{ FROM unsalted t1 JOIN unsalted t2 }}
{{ ON (t1.keyA = t2.keyA) }}
{{ GROUP BY t1.val, t2.val;}}
{{ +-------------------------------------------------------------+----------------++------------------+}}{{}}
|PLAN|EST_BYTES_READ|EST_ROWS_READ| |

{{}}{{+-------------------------------------------------------------+----------------++------------------+}}{{}}
|SORT-MERGE-JOIN (INNER) TABLES|null|null| |
|CLIENT 1-CHUNK PARALLEL 1-WAY FULL SCAN OVER UNSALTED|null|null| |
|AND|null|null| |
|CLIENT 1-CHUNK PARALLEL 1-WAY FULL SCAN OVER UNSALTED|null|null| |
|CLIENT SORTED BY [TO_DECIMAL(T1.VAL), T2.VAL]|null|null| |
|CLIENT AGGREGATE INTO DISTINCT ROWS BY [T1.VAL, T2.VAL]|null|null| |

{{}}{{+-------------------------------------------------------------+----------------++------------------+}}



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