检查数据倾斜分布

从传统数据库迁移到GP中一个重要的且常常被开发者忽略的概念是数据分布，没有良好的设计表的分布键会导致严重的性能问题。下面函数将给开发者及DBA检測一个表的数据倾斜情况。

-- Function: gpmg.data_skew(character varying)

-- DROP FUNCTION gpmg.data_skew(character varying);

CREATE OR REPLACE FUNCTION gpmg.data_skew(tablename character varying)
  RETURNS text AS
$BODY$
--2014-05-26,Gtlions,收集和统计数据倾斜情况
declare
  v_func character varying(200)='gpmg.data_skew()';
  v_begin_time timestamp;
  v_end_time timestamp;
  v_status int=0;
  v_msg text='Done.';
  v_record record;

  v_id integer;
  v_rq timestamp;
  v_segs integer=64;
  v_totalnums bigint=0;
  v_maxskew numeric=0.0;
  v_minskew numeric=0.0;
  v_maxskew_seg varchar(20);
  v_minskew_seg varchar(20);
  v_maxrows bigint=0;
  v_minrows bigint=0;
  v_result varchar(2000);

begin
  v_id=nextval('gpmg.commonseq');
  v_rq=now();
  v_begin_time=clock_timestamp();
  v_result = 'GP hava ';
  select into v_segs count(*) segs from gp_segment_configuration where role='p' and content<>-1;
  v_result = v_result||v_segs||' instances, Standard skew is '||1.0/v_segs||'. ';
  -- bg1 segid, bg2 节点记录数量
  execute 'insert into gpmg.commontab(seq,tabname,bg1,bg2) select '||v_id||','''||$1||''',gp_segment_id,count(*) segrownums from '||$1||' group by rollup(( gp_segment_id)) order by gp_segment_id';
  select into v_segs,v_totalnums v_segs,max(bg2) from gpmg.commontab where seq=v_id and tabname=$1;
  --nm1 标准倾斜率, nm2 节点倾斜率, nm3 标准-节点倾斜率绝对值
  update gpmg.commontab set nm1=1::numeric/v_segs,nm2=bg2::numeric/v_totalnums,nm3=abs(1::numeric/v_segs-bg2::numeric/v_totalnums) where seq=v_id and tabname=$1;
  select into v_maxskew,v_minskew max(nm2),min(nm2) from gpmg.commontab where seq=v_id and tabname=$1 and bg1 is not null;

  select into v_maxskew_seg hostname from gp_segment_configuration where role='p' and content in (select bg1 from gpmg.commontab where seq=v_id and tabname=$1 and bg1 is not null and nm2=v_maxskew limit 1);
  select into v_minskew_seg hostname from gp_segment_configuration where role='p' and content in (select bg1 from gpmg.commontab where seq=v_id and tabname=$1 and bg1 is not null and nm2=v_minskew limit 1);

  select into v_maxrows bg2 from gpmg.commontab where seq=v_id and tabname=$1 and bg1 is not null and nm2=v_maxskew limit 1;
  select into v_minrows bg2 from gpmg.commontab where seq=v_id and tabname=$1 and bg1 is not null and nm2=v_minskew limit 1;

  v_result =v_result ||'You Table ['||$1||'] skew info: [table_totalrows:'||v_totalnums||', maxskew:seg-'||v_maxskew_seg||', rows-'||v_maxrows||' '||v_maxskew||', minskew:seg-'||v_minskew_seg||', rows-'||v_minrows||' '||v_minskew||']';
  delete from gpmg.commontab where seq=v_id and tabname=$1;
  return v_result;
  v_end_time=clock_timestamp();
end;
$BODY$
  LANGUAGE plpgsql VOLATILE;
ALTER FUNCTION gpmg.data_skew(character varying)
  OWNER TO gpadmin;
GRANT EXECUTE ON FUNCTION gpmg.data_skew(character varying) TO public;
GRANT EXECUTE ON FUNCTION gpmg.data_skew(character varying) TO gpadmin;

bigdatagp=# select gpmg.data_skew('gpmg.manager_table');
                                                                                                            data_skew                                                  

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------
 GP hava 64 instances, Standard skew is 0.01562500000000000000. You Table [gpmg.manager_table] skew info: [table_totalrows:83, maxskew:seg-sdw16, rows-3 0.036144578313
25301205, minskew:seg-sdw2, rows-1 0.01204819277108433735]
(1 row)

bigdatagp=# select gpmg.data_skew('gpmg.func_log');
                                                                                                             data_skew                                                 

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------
 GP hava 64 instances, Standard skew is 0.01562500000000000000. You Table [gpmg.func_log] skew info: [table_totalrows:53708, maxskew:seg-sdw10, rows-907 0.016887614508
08073285, minskew:seg-sdw7, rows-773 0.01439264169211290683]
(1 row)
2014-10-14 09:53:00

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