planner: extract a LogicalMemTable from DataSource to decouple memory/stored tables

[lonng]

Signed-off-by: Lonng heng@lonng.org

What problem does this PR solve?

In the current implementation, all tables will be represented as a DataSource in the logical plan build stage. But tables can be classified into the virtual table (memory table which data are stored in memory) and the stored table (data are stored in TiKV). And in our #13567 feature need to push some predictions down to the virtual table to make data retrieval more efficiency.

e.g:

SELECT * FROM tidb_cluster_log WHERE type='tikv' AND address='192.16.5.32'

In the current implementation, we must retrieve all logs from cluster components and filter them in TiDB by predications type='tikv' AND address='192.16.5.32', which is inefficiency. And the more efficient way is that push type='tikv' AND address='192.16.5.32' to the lowest virtual table tidb_cluster_log and make the LogReaderExec have some information about predications.

This PR split DataSource into DataSource and LogicalMemTable and make the LogicalMemTable independent to make the subsequent work easily and don't use any tricky/hack ways to achieve our goal.

What is changed and how it works?

This PR extracts a LogicalMemTable from DataSource to decouple memory/stored tables and make some predications push-down to memory table more easily.

Check List

Tests

• Exists Unit test

Release note

• No need

[codecov]

Codecov Report

Merging #13741 into master will not change coverage.
The diff coverage is n/a.

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  Hits          94436      94436           
  Misses        15840      15840           
  Partials       7355       7355

[winoros]

lgtm

[crazycs520]

LGTM

[crazycs520]

/run-all-tests

[crazycs520]

/run-all-tests