Greenplum distribution

Question

There is a table with a random distribution

CREATE TABLE schema.table (
    col1 int4 NULL,
    col2 int4 NULL,
    col3 int4 NULL
)
WITH (
    appendonly=true,
    compresstype=zstd,
    orientation=column
)
DISTRIBUTED RANDOMLY; 

We need to optimally (with minimal skew) distribute rows over one field. For this we can create test tables

CREATE TABLE schema.test_table (
    col_1 int4 NULL,
    col_2 int4 NULL,
    col_3 int4 NULL
)
WITH (
    appendonly=true,
    compresstype=zstd,
    orientation=column
)
DISTRIBUTED BY (col_i); 
INSERT INTO schema.test_table SELECT * FROM schema.table;

And then check them against skew, for example via

select * from gp_toolkit.gp_skew_coefficient('schema.test_table'::regclass);

The problem is that we want to check the table for skew without creating test tables. Can this be done, and if so, how?

Answer 1

Something like this will work if you really do not want to create a new table, but I do not know of a way to model the distribution without actually (re)distributing the data.

foo=# create table foo(a int, b int, c int);
NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'a' as the Greenplum Database data distribution key for this table.
HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.
CREATE TABLE

foo=# insert into foo values (generate_series(1,100), generate_series(101,200), generate_series(2001, 2100));
INSERT 0 100

foo=# select * from gp_toolkit.gp_skew_coefficient('public.foo'::regclass);
 skcoid |         skccoeff
--------+--------------------------
  76788 | 18.460769214742921763000
(1 row)

foo=# select gp_segment_id, count(*) from foo group by 1 order by 1;
 gp_segment_id | count
---------------+-------
             0 |    17
             1 |    18
             2 |    23
             3 |    17
             4 |    15
             5 |    10
(6 rows)


foo=# ALTER TABLE foo SET
foo-# WITH (REORGANIZE=true)
foo-# DISTRIBUTED BY (a);
ALTER TABLE
foo=# select * from gp_toolkit.gp_skew_coefficient('public.foo'::regclass);
 skcoid |         skccoeff
--------+--------------------------
  76788 | 18.460769214742921763000
(1 row)

foo=# select gp_segment_id, count(*) from foo group by 1 order by 1;
 gp_segment_id | count
---------------+-------
             0 |    21
             1 |    18
             2 |    12
             3 |    15
             4 |    18
             5 |    16
(6 rows)

foo=#

foo=# ALTER TABLE foo SET
WITH (REORGANIZE=true)
DISTRIBUTED BY (b);
ALTER TABLE
foo=# select * from gp_toolkit.gp_skew_coefficient('public.foo'::regclass);
 skcoid |         skccoeff
--------+--------------------------
  76788 | 27.011108825814611346000
(1 row)

foo=# select gp_segment_id, count(*) from foo group by 1 order by 1;
 gp_segment_id | count
---------------+-------
             0 |    12
             1 |    14
             2 |    20
             3 |    24
             4 |    16
             5 |    14
(6 rows)

foo=#


foo=# ALTER TABLE foo SET
WITH (REORGANIZE=true)
DISTRIBUTED BY (c);
ALTER TABLE
foo=# select * from gp_toolkit.gp_skew_coefficient('public.foo'::regclass);
 skcoid |         skccoeff
--------+--------------------------
  76788 | 30.983866769659334938000
(1 row)

foo=#

foo=# select gp_segment_id, count(*) from foo group by 1 order by 1;
 gp_segment_id | count
---------------+-------
             0 |    19
             1 |    10
             2 |    20
             3 |    23
             4 |    11
             5 |    17
(6 rows)

foo=#