enhance partition pruning for table partition

[tiancaiamao]

There are two known issues for current partition pruning implementation:

1. it doesn't support filter conditions that can't be push down to datasource
2. it doesn't support expressions that can't calculate range

Both of them are not reasonable.

@coocood advised in #6642 to ignore the selection node above datasoure node, so filter conditions that can't be push down to datasource won't be considered by partition pruning function, that's why limitation 1 exists.

Suppose we haven't implement timestamp type in TiKV yet, the filter condition will not be pushed down to datasource, then partition pruning can't work on this example:

select ts from t where ts > 20060308110502

Limitation 2 comes from the fact that we currently use range calculation to implement partition pruning.

create table t (id int) partition by range (to_days(id)) ...
select * from t where id > 2018-03-08

to_days is a function, we can't calculate range for it, so partition pruning can't work.

[winoros]

This is also the limitation the non-partitioned table have when calculating range.

[tiancaiamao]

A demo for it https://gist.github.com/tiancaiamao/4aab62ba70381fce15da1e4572fbd0b1

[morgo]

In #7523 I have a small feature request which complements test-cases for partition pruning.

[tiancaiamao]

I have already make partition info visible in this PR:
https://github.com/pingcap/tidb/pull/7505/files#diff-0d9c730193ced8057a092d0c770ea44fR28

There is a lot of room for improvement, any suggestion is welcome. @morgo

[tiancaiamao]

PR #7643 has been merged.
I'll update this issue soon.

[nanne007]

Awesome