Optimize count(*) with table statistics

[Dandandan]

Which issue does this PR close?

Closes #618

Rationale for this change

Speeding up queries (or part of a query) that can utilize the availability of the number of rows in the table statistics. Often used to inspect tables.
Not having to scan the data can save considerable time / cost.

On the TPC-H lineitem table we can see a 80x speed up:

> CREATE EXTERNAL TABLE X STORED AS PARQUET LOCATION '../benchmarks/parquet/lineitem/';
> select count(*) from X;
+------------------+
| COUNT(Uint8(1)) |
+------------------+
| 6001214          |
+------------------+
1 row in set. Query took 0.001 seconds.

Master:

> CREATE EXTERNAL TABLE X STORED AS PARQUET LOCATION '../benchmarks/parquet/lineitem/';
> select count(*) from X;
+-----------------+
| COUNT(UInt8(1)) |
+-----------------+
| 6001214         |
+-----------------+
1 row in set. Query took 0.081 seconds.

What changes are included in this PR?

Are there any user-facing changes?

[alamb]

Looks great -- thank you @Dandandan.

I have filed https://github.com/influxdata/influxdb_iox/issues/1815 to get IOx using this upstream. Great stuff.

[alamb]

The nice thing about adding a has_exact_statistics is that it is a backwards compatible API

An alternate might be to encapsulate the "Exact statistics or not" into a field on Statistics itself, which feels to me like it keeps related things together more, but has the downside of changing Statistics / APIs

[Dandandan]

That's a similar thought process I had.
Maybe at some point it would also be nice to tell what parts of the statistics are exact (e.g. number of rows) and what estimated (such as distinct count).

[alamb]

❤️

I think we need to check for empty gby as well

[Dandandan]

After this PR I also plan some follow up issues:

• also supporting min/max statistics

[houqp]

Good stuff 👍 I recommend using logical plan builder to construct the new plans to make things more robust.

[alamb]

This is a cool optimization 👍

[alamb]

Good stuff 👍 I recommend using logical plan builder to construct the new plans to make things more robust.

Perhaps we can do this as a follow on PR