[CH-45]Support count(1)/count(*)

[lgbo-ustc]

Relate to the issue https://github.com/Kyligence/ClickHouse/issues/45

What changes were proposed in this pull request?

A empty base_schema will be made in the scan node when run the following queries

select count(1) from t;
select count(*) from t;
select sum(2) from t;
select 1 from t;

The backend source node will return empty data or throw an exception when there is no column to read. So we fill a attribute to read in FileSourceScanExec.

Another problem is in ProjectExecTransformer. If ProjectExecTransformer's child is an ColumnarInputAdapter and projectList is empty, it would generate a scan node with empty base_schema. So when childCtx is null, we don't return directly.

How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

[github-actions[bot]]

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See also:

• Other pull requests

[zzcclp]

Please rebase to main and add some UTs

[zzcclp]

Please rebase to main

[lgbo-ustc]

wait for https://github.com/Kyligence/ClickHouse/pull/175 be merged