[DISCUSSION] Order By Dimension ＋ Limit Optimizations

[马云 <si...@163.com>]

Hi Carbon Dev,


currently  I have done optimization for ordering by 1 dimension.
my local performance test as below. Please give your suggestion.
my optimization solution for order by 1 dimension as below
mainly filter some unnecessary  blocklets and leverage  the dimension's order stored feature to get sorted data in each partition.
at last use the TakeOrderedAndProject to merge sorted data from partitions
step1. change logical plan and push down the order by and limit information to carbon scan
            and change sort physical plan to TakeOrderedAndProject  since data will be get and sorted in each partition
step2. in each partition apply the limit number, blocklet's min_max index to filter blocklet. 
          it can reduce scan data if some blocklets were filtered 
         for example,  SELECT name, serialname, country, salary, id, date FROM t3 ORDER BY serialname limit 10000
 supposing there are 2 blocklets , each has 32000 data, serial name  is between serialname1 to serialname2 in the first blocklet 
and between  serialname2 to serialname3 in the second blocklet. actually we only need to scan the first blocklet 
since 32000 > 100 and first blocklet's serial name <= second blocklet's serial name


step3.  load the order by dimension data to  scanResult.  put all scanResults to a TreeSet for sorting
              Other columns' data will be lazy-loaded in step4.
step4. according to the limit value, use a iterator to get the topN sorted data from the TreeSet. In the same time to load other columns data if needed. 
           in this step  it tries to reduce scanning non-sort dimension  data.
         for example,  SELECT name, serialname, country, salary, id, date FROM t3 ORDER BY serialname limit 10000
 supposing there are 3 blocklets ,  in the first 2 blocklets, serial name  is between serialname1 to serialname100 and each has 2500 serialname1 and serialname2.
In the third blocklet, serial name  is between serialname2 to serialnam100, but no serialname1 in it.
load serial name data for the 3 blocklets and put all to a treeset sorting by the  min serialname.
apparently use  iterator to get the top 10000 sorted data, it only need to care the first 2 blocklets(5000 serialname1 + 5000 serialname2).
In others words,  it  loads serial name data for the 3 blocklets.But only "load name, country, salary, id, date"'s data for the first 2 blocklets


step5. TakeOrderedAndProject physical plan will be used to merge sorted data from partitions 


the below items also can be optimized in future


leverage  mdk keys' order feature to optimize the SQL who order by prefix dimension columns of MDK
use the dimension order feature in blocklet lever and dimensions' inverted index to optimize SQL who order by multi-dimensions