Re: Introduce Index Aggregate - new GROUP BY strategy

Hi!

> Are you able to provide benchmarks
Yes, sure.

Test matrix:

- number of groups: from 100 to 1000000 increased by 10 times
- different types: int, bigint, uuid, text
- strategy: hash, group, index

For each key value there are 3 tuples with different 'j' value (for
aggregation logic).

Also, there is a test (called bigtext) for large string as a key (each string is 4kB).

To test pgbench is used. Test query looks like this:

     select i, sum(j) from TBL group by 1 order by 1;

Depending on the table size duration is set from 1 to 3 minutes.
Everything in attached scripts:

- setup.sql - script to setup environment (create tables, setup GUCs).
                      after running this you should restart database.
                      NOTE: actually, for int and bigint number of groups is less
                                  than power of 10
- run_bench.sh - shell script that runs test workload. After running
                               it will create files with pgbench results.
- collect_results.sh - parses output files and formats result table.
                                     As values it shows TPS.
- show_plan.sh - small script to run EXPLAIN for each run query

Finally, I have this table:

int

| amount  | HashAgg       | GroupAgg       | IndexAgg     |
| ------------- | ------------------ | ------------------- | ------------------ |
| 100          | 3249.929602 | 3501.174072 | 3765.727121 |
| 1000       | 504.420643   | 501.465754    | 575.255906   |
| 10000     | 50.528155     | 49.312322      | 54.510261     |
| 100000   | 4.775069       | 4.317584        | 4.791735       |
| 1000000 | 0.405538       | 0.406698        | 0.321379       |

bigint

| amount   | HashAgg       | GroupAgg     | IndexAgg       |
| ------------  | -------------------| ------------------- | ------------------  |
| 100          | 3225.287886 | 3510.612641 | 3742.911726 |
| 1000        | 492.908092   | 491.530184   | 574.475159   |
| 10000      | 50.192018     | 49.555983     | 53.909437     |
| 100000    | 4.831086       | 4.430059       | 4.748821       |
| 1000000  | 0.401983       | 0.413218       | 0.318144       |

text

| amount  | HashAgg       | GroupAgg     | IndexAgg       |
| ------------ | -------------------| ------------------- | ------------------ |
| 100         | 2647.030876 | 2553.503954 | 2946.282525 |
| 1000       | 348.464373   | 286.818555   | 342.771923   |
| 10000     | 32.891834     | 24.386304     | 28.249571      |
| 100000   | 2.934513       | 1.956983       | 2.237997        |
| 1000000 | 0.249291       | 0.148780       | 0.150943        |

uuid

| amount  | HashAgg      | GroupAgg       | IndexAgg      |
| ------------ | ------------------ | ------------------- | ------------------  |
| 100         | N/A                 | 2282.812585 | 2432.713816 |
| 1000       | N/A                 | 282.637163   | 303.892131    |
| 10000     | N/A                 | 28.375838     | 28.924711      |
| 100000   | N/A                 | 2.649958       | 2.449907 |
| 1000000 | N/A                 | 0.255203       | 0.194414        |

bigtext

| HashAgg  | GroupAgg | IndexAgg |
| -------------- | --------------- | -------------- |
| N/A            | 0.035247   | 0.041120  |

NOTES: I could not make Hash + Sort plan for uuid and bigtext
               test and it reproduces even on upstream without this patch.

The main observation is that on small amount of groups
Index Aggregate performs better than other strategies:

- int and bigint even up to 100K keys
- text only for 100 keys
- uuid up to 10K keys
- bigtext better than Group + Sort, but tested only on big amount
    of keys (100K)

---
Sergey Soloviev

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