Hash segmentation to node number mapping

RaghavARaghavA Registered User
edited November 27 in Vertica Forum

Hi,

Does anyone can help to explain how vertica maps node_name with the Hash segmentation?

In the following example, i created a dummy table and inserted few rows in it. Then, we can see that all the rows are not equally distributed between all nodes. Distribution of data across node happens according to HASH(ID) value.

I need to understand how vertica maps/decides that which hash value will be stored in which node?

    Sample Table:

    dbadmin=> CREATE TABLE checkout (
    dbadmin(>     id INT ENCODING RLE,
    dbadmin(>     transaction_timestamp TIMESTAMP ENCODING DELTAVAL,
    dbadmin(>     transaction_amount DOUBLE PRECISION
    dbadmin(> )
    dbadmin-> ORDER BY transaction_timestamp
    dbadmin-> SEGMENTED BY hash(id) ALL NODES;
    CREATE TABLE


    dbadmin=> select count(*) from checkout;
     count
    -------
        16
    (1 row)

    dbadmin=> select local_node_name(), count(*) from checkout group by local_node_name();
     local_node_name | count
    -----------------+-------
     v_vdb_node0001  |     5
     v_vdb_node0002  |     9
     v_vdb_node0003  |     2
    (3 rows)

    dbadmin=> select id, hash(id), transaction_timestamp, transaction_amount from checkout order by 1;
     id |        hash         |   transaction_timestamp    | transaction_amount
    ----+---------------------+----------------------------+--------------------
      1 | 5783548743464686114 | 2018-11-27 04:13:41.157426 |                100
      2 | 1618211815126016456 | 2018-11-29 04:13:45.306561 |                200
      3 | 3883506187811235133 | 2018-11-30 04:14:02.686752 |                300
      4 | 5130139942120141781 | 2018-12-01 04:14:08.806913 |                400
      5 | 8299427032879314813 | 2018-12-02 04:14:14.25534  |                500
      5 | 8299427032879314813 | 2018-12-02 04:17:34.520367 |                500
      6 | 3939990413061782359 | 2018-12-03 04:17:34.551429 |                600
      7 | 3110550038591389753 | 2018-12-04 04:17:34.580134 |                700
      8 | 9046158759674539325 | 2018-12-05 04:17:34.608005 |                800
      9 | 6898810103076931951 | 2018-12-06 04:17:34.636077 |                900
     10 | 1909575676721759820 | 2018-12-07 04:17:34.664295 |               1000
     11 | 2646627613614353023 | 2018-12-08 04:17:34.692378 |               1100
     12 | 6042820943135784349 | 2018-12-09 04:17:34.720272 |               1200
     13 | 2568111303865075193 | 2018-12-10 04:17:34.748703 |               1300
     14 | 4970994696257251037 | 2018-12-11 04:17:34.7767   |               1400
     15 | 6850683294198565620 | 2018-12-12 04:17:36.528584 |               1500
    (16 rows)

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