PGXC_GET_TABLE_SKEWNESS

PGXC_GET_TABLE_SKEWNESS displays the data skew on tables in the current database. Only the system administrator or the preset role gs_role_read_all_stats can access this view.

**Table 1** PGXC_GET_TABLE_SKEWNESS columns
Column	Type	Description
schemaname	name	Schema name of a table
tablename	name	Name of a table
totalsize	numeric	Total size of a table, in bytes
avgsize	numeric(1000,0)	Average table size (total table size divided by the number of DNs), which is the ideal size of tables distributed on each DN
maxratio	numeric(10,3)	Ratio of the maximum table size on a single DN to avgsize
minratio	numeric(10,3)	Ratio of the minimum table size on a single DN to avgsize
skewsize	bigint	Table skew rate (the maximum table size on a single DN minus the minimum table size on a single DN)
skewratio	numeric(10,3)	Table skew rate (skewsize/avgsize)
skewstddev	numeric(1000,0)	Standard deviation of table distribution (For two tables of the same size, a larger deviation indicates a more severe skew.)

Example

Query the data skew status of all tables in the current database (the number of tables in the database is less than 10,000).

    
         SELECT * FROM pgxc_get_table_skewness ORDER BY totalsize DESC;
 schemaname |        tablename        | totalsize | avgsize | maxratio | minratio | skewsize | skewratio | skewstddev
------------+-------------------------+-----------+---------+----------+----------+----------+-----------+------------
 dbadmin    | reason                  |    147456 |   49152 |     .333 |     .333 |        0 |     0.000 |          0
 tpcds      | reason_t2               |     73728 |   24576 |     .556 |    0.000 |    40960 |      .556 |      21674
 dbadmin    | reason_bk               |     65536 |   21845 |     .500 |    0.000 |    32768 |      .500 |      18919
 tsearch    | pgweb                   |     49152 |   16384 |     .333 |     .333 |        0 |     0.000 |          0
 dbadmin    | student                 |     40960 |   13653 |     .400 |     .200 |     8192 |      .200 |       4730
 tsearch    | ts_zhparser             |     40960 |   13653 |     .400 |     .200 |     8192 |      .200 |       4730
 dbms_om    | gs_wlm_session_info     |     24576 |    8192 |     .333 |     .333 |        0 |     0.000 |          0
 dbms_om    | gs_wlm_ec_operator_info |     24576 |    8192 |     .333 |     .333 |        0 |     0.000 |          0
 dbms_om    | gs_wlm_operator_info    |     24576 |    8192 |     .333 |     .333 |        0 |     0.000 |          0
(9 rows)

If the number of tables in the database is more than 10,000, do not use the PGXC_GET_TABLE_SKEWNESS view because it takes a long time (hours) to query the entire database for skewed columns. You are advised to refer to the definition of the PGXC_GET_TABLE_SKEWNESS view and use the table_distribution() function to define the output. This optimizes the calculation by reducing the output columns. An example is shown as follows:

     
          SELECT schemaname,tablename,max(dnsize) AS maxsize, min(dnsize) AS minsize 
FROM pg_catalog.pg_class c 
INNER JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace 
INNER JOIN pg_catalog.table_distribution() s ON s.schemaname = n.nspname AND s.tablename = c.relname 
INNER JOIN pg_catalog.pgxc_class x ON c.oid = x.pcrelid AND x.pclocatortype = 'H' 
GROUP BY schemaname,tablename;