Improve aggregate performance by special casing single group keys

[alamb]

Is your feature request related to a problem or challenge?

This is another observation to make aggregation queries go faster

After #6904 the speed of grouping is much faster as much of the per-aggregate state overhead is removed.

Now, for queries with a single primitive type (e.g. Int64) a significant portion of their execution time is the converting the grouping key into the the arrow Row format: https://docs.rs/arrow-row/43.0.0/arrow_row/index.html to compare

An example of such a query is to find the number of interactions with some user id:

SELECT count(*)
FROM t
GROUP BY user_id

Describe the solution you'd like

As @Dandandan suggested on #6800 (comment)

we should also consider avoiding the conversion to the row-format as this likely will be one of the more expensive things now.

So that would mean that for group by with a single column, we could use the (native) representation of that column to store the group values rather than using the Arrow Row format.

This is the same approach taken today by Sort / SortPreservingMerge. It is implemented via a generic FieldCursor:

https://github.com/apache/arrow-datafusion/blob/d316702722e6c301fdb23a9698f7ec415ef548e9/datafusion/core/src/physical_plan/sorts/cursor.rs#L180-L191

Describe alternatives you've considered

@yahoNanJing has also been exploring a fixed width row cursor implementation that is optimized for comparison rather than ordering: apache/arrow-rs#4524

Additional context

There is some discussion in the ASF slack channel as well: https://the-asf.slack.com/archives/C01QUFS30TD/p1689301661826909

[alamb]

@yahoNanJing reports that with apache/arrow-rs#4523 / apache/arrow-rs#4524 and yahoNanJing@a18ac07 they see a significant performance improvement in TPHC queries

Thus, I think this idea holds significant promise

[alamb]

Just to be clear, what I was imagining for the group storage is not to change the contents of the RawTable (it will continue to contain group_indexes).

But instead of storing group_values using the arrow Row format

 stores "group       stores group values, 
    indexes"          in arrow_row format 
                                          
 ┌─────────────┐      ┌────────────┐      
 │   ┌─────┐   │      │ ┌────────┐ │      
 │   │  5  │   │ ┌────┼▶│  "A"   │ │      
 │   ├─────┤   │ │    │ ├────────┤ │      
 │   │  9  │   │ │    │ │  "Z"   │ │      
 │   └─────┘   │ │    │ └────────┘ │      
 │     ...     │ │    │            │      
 │   ┌─────┐   │ │    │    ...     │      
 │   │  1  │───┼─┘    │            │      
 │   ├─────┤   │      │            │      
 │   │ 13  │───┼─┐    │ ┌────────┐ │      
 │   └─────┘   │ └────┼▶│  "Q"   │ │      
 └─────────────┘      │ └────────┘ │      
                      │            │      
                      └────────────┘      
                                          
                                          
                                          
       map            group_values        
  (Hash Table)                            
                                                                                                
                                                                               

We would instead store the group values using a native type like Vec<T> like this

 stores "group               stored in a      
    indexes"                native Vec<T>     
                                              
 ┌─────────────┐            ┌──────────┐      
 │   ┌─────┐   │            │ ┌──────┐ │      
 │   │  5  │   │    ┌───────┼▶│  1   │ │      
 │   ├─────┤   │    │       │ ├──────┤ │      
 │   │  9  │   │    │       │ │  3   │ │      
 │   └─────┘   │    │       │ └──────┘ │      
 │     ...     │    │       │          │      
 │   ┌─────┐   │    │       │    ...   │      
 │   │  1  │───┼────┘       │          │      
 │   ├─────┤   │            │          │      
 │   │ 13  │───┼────┐       │ ┌──────┐ │      
 │   └─────┘   │    └───────┼▶│  5   │ │      
 └─────────────┘            │ └──────┘ │      
                            │          │      
                            └──────────┘      
                                              
                                              
                            group_values      
       map                                    
  (Hash Table)                                
                                              

Handling null values would need some care, but since this would only be for single columns (where there can be at most one null value) I think we could figure out some way to handle it

[alamb]

I ran some some tests of the various approaches on

• fixed width row format, inlined into hash table: https://github.com/yahoNanJing/arrow-datafusion/tree/EBAY-KYLIN-4003-5 from @yahoNanJing (aka with the change yahoNanJing@a18ac07 embeds the group value directly into the table)

• unsafe row access Test: inline everything and add row_unchecked arrow-rs#4539 / Test: Use unchecked group_value access in group hash #7010

EBAY-KYLIN-4003-5 (inlined group values): mixed results

--------------------
Benchmark tpch_mem.json
--------------------
┏━━━━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Query        ┃ main_base ┃ EBAY-KYLIN-4003-5 ┃        Change ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ QQuery 1     │  537.94ms │          544.54ms │     no change │
│ QQuery 2     │  161.75ms │          169.20ms │     no change │
│ QQuery 3     │  164.01ms │          169.22ms │     no change │
│ QQuery 4     │  114.47ms │          112.47ms │     no change │
│ QQuery 5     │  377.65ms │          389.09ms │     no change │
│ QQuery 6     │   41.12ms │           39.79ms │     no change │
│ QQuery 7     │  885.44ms │          882.59ms │     no change │
│ QQuery 8     │  237.45ms │          234.45ms │     no change │
│ QQuery 9     │  556.79ms │          556.93ms │     no change │
│ QQuery 10    │  313.01ms │          326.68ms │     no change │
│ QQuery 11    │  158.40ms │          168.29ms │  1.06x slower │
│ QQuery 12    │  171.57ms │          161.69ms │ +1.06x faster │
│ QQuery 13    │  298.77ms │          287.11ms │     no change │
│ QQuery 14    │   49.25ms │           47.53ms │     no change │
│ QQuery 15    │   49.57ms │           50.07ms │     no change │
│ QQuery 16    │  167.13ms │          171.66ms │     no change │
│ QQuery 17    │  950.03ms │          871.42ms │ +1.09x faster │
│ QQuery 18    │ 1576.97ms │         1706.98ms │  1.08x slower │
│ QQuery 19    │  178.84ms │          161.94ms │ +1.10x faster │
│ QQuery 20    │  331.30ms │          293.13ms │ +1.13x faster │
│ QQuery 21    │ 1120.20ms │         1055.25ms │ +1.06x faster │
│ QQuery 22    │   84.74ms │           86.40ms │     no change │
└──────────────┴───────────┴───────────────────┴───────────────┘

Unsafe Row Access: mixed results

--------------------
Benchmark tpch_mem.json
--------------------
┏━━━━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Query        ┃ main_base ┃ alamb_peephole ┃        Change ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ QQuery 1     │  548.62ms │       519.95ms │ +1.06x faster │
│ QQuery 2     │  154.00ms │       149.26ms │     no change │
│ QQuery 3     │  159.95ms │       164.44ms │     no change │
│ QQuery 4     │  113.52ms │       117.78ms │     no change │
│ QQuery 5     │  374.59ms │       381.38ms │     no change │
│ QQuery 6     │   39.37ms │        43.51ms │  1.11x slower │
│ QQuery 7     │  932.03ms │       947.92ms │     no change │
│ QQuery 8     │  231.40ms │       245.70ms │  1.06x slower │
│ QQuery 9     │  569.26ms │       551.04ms │     no change │
│ QQuery 10    │  325.29ms │       321.81ms │     no change │
│ QQuery 11    │  171.33ms │       156.11ms │ +1.10x faster │
│ QQuery 12    │  167.87ms │       158.86ms │ +1.06x faster │
│ QQuery 13    │  302.12ms │       282.03ms │ +1.07x faster │
│ QQuery 14    │   47.34ms │        47.49ms │     no change │
│ QQuery 15    │   52.53ms │        52.89ms │     no change │
│ QQuery 16    │  153.12ms │       149.93ms │     no change │
│ QQuery 17    │  914.18ms │       977.18ms │  1.07x slower │
│ QQuery 18    │ 1527.32ms │      1612.21ms │  1.06x slower │
│ QQuery 19    │  164.35ms │       152.92ms │ +1.07x faster │
│ QQuery 20    │  308.12ms │       312.37ms │     no change │
│ QQuery 21    │ 1047.68ms │      1080.79ms │     no change │
│ QQuery 22    │   88.36ms │        85.59ms │     no change │
└──────────────┴───────────┴────────────────┴───────────────┘