[GitHub] [arrow-datafusion] yjshen commented on issue #5325: Optimize Accumulator `size` function performance (fix regression on clickbench)

["yjshen (via GitHub)" <gi...@apache.org>]

yjshen commented on issue #5325:
URL: https://github.com/apache/arrow-datafusion/issues/5325#issuecomment-1444994728

   We could rewrite an aggregate query with distinct aggregations into an expanded double aggregation, as Spark did:
   
   Before
   ```
   Aggregate(
      key = ['key]
      functions = [COUNT(DISTINCT 'cat1),
                   COUNT(DISTINCT 'cat2),
                   sum('value)]
      output = ['key, 'cat1_cnt, 'cat2_cnt, 'total])
     LocalTableScan [...]
   ```
   
   After rewrite:
   ```
   Aggregate(
      key = ['key]
      functions = [count('cat1) FILTER (WHERE 'gid = 1),
                   count('cat2) FILTER (WHERE 'gid = 2),
                   first('total) ignore nulls FILTER (WHERE 'gid = 0)]
      output = ['key, 'cat1_cnt, 'cat2_cnt, 'total])
     Aggregate(
        key = ['key, 'cat1, 'cat2, 'gid]
        functions = [sum('value)]
        output = ['key, 'cat1, 'cat2, 'gid, 'total])
       Expand(
          projections = [('key, null, null, 0, cast('value as bigint)),
                         ('key, 'cat1, null, 1, null),
                         ('key, null, 'cat2, 2, null)]
          output = ['key, 'cat1, 'cat2, 'gid, 'value])
         LocalTableScan [...]
   ```
   
   


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