*: fix union scan for partitioned table

[tiancaiamao]

What problem does this PR solve?

MySQL [test]> show create table t1;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                          |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `c1` smallint(6) NOT NULL,
  `c2` char(5) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY RANGE ( `c1` ) (
  PARTITION p0 VALUES LESS THAN (10),
  PARTITION p1 VALUES LESS THAN (20),
  PARTITION p2 VALUES LESS THAN (30),
  PARTITION p3 VALUES LESS THAN (MAXVALUE)
) |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MySQL [test]> begin;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> insert into t1 values (1,1);
Query OK, 1 row affected (0.00 sec)

MySQL [test]> select * from t1;
Empty set (0.01 sec)

What is changed and how it works?

How does union scan works for partitioned table:

1. In rule_partition_processor.go, the rewrite rule changes

UnionScan->DataSource

to

UnionAll{UnionScan->Partition1, UnionScan->Partition2 ...}

Note that UnionScan is now the parent of every Partition, rather than the previous

UnionScan->UnionAll{Partition1, Partition2 ...}

2. UnionScan->Partition1 transformed to UnionScanExec+three reader, the reader read data from partition, and UnionExec get the incremental buffer changes on partition, not table

3. To get the incremental buffer changes on partition:

• The id should be physicalID, instead of tableID
• Dirty table is organised using partition, i.e, DirtyDB = map(physicalID -> changes)

4. In the three kinds of reader(TableReader,IndexReader,IndexLookup) struct, table is also changed to partition (PhysicalTable)

5. The function signature StmtAddDirtyTableOP should be

StmtAddDirtyTableOP(op int, physicalID int64, handle int64, row []types.Datum)

instead of

StmtAddDirtyTableOP(op int, tid int64, handle int64, row []types.Datum)

---

This change is 

[tiancaiamao]

PTAL @lamxTyler @zz-jason

[tiancaiamao]

PTAL @zz-jason @lamxTyler

[zz-jason]

@tiancaiamao any update?

[codecov-io]

Codecov Report

Merging #8871 into master will increase coverage by 0.01%.
The diff coverage is 83.67%.

@@            Coverage Diff             @@
##           master    #8871      +/-   ##
==========================================
+ Coverage    67.4%   67.42%   +0.01%     
==========================================
  Files         373      373              
  Lines       78544    78593      +49     
==========================================
+ Hits        52946    52990      +44     
- Misses      20838    20844       +6     
+ Partials     4760     4759       -1

Impacted Files	Coverage Δ
table/tables/tables.go	52.41% <0%> (ø)	⬆️
executor/builder.go	83.91% <100%> (+0.11%)	⬆️
executor/union_scan.go	78.61% <25%> (-0.13%)	⬇️
planner/core/rule_partition_processor.go	74.19% <75%> (+0.16%)	⬆️
util/systimemon/systime_mon.go	80% <0%> (-20%)	⬇️
ddl/ddl_worker.go	87.65% <0%> (-0.65%)	⬇️
sessionctx/variable/varsutil.go	25.45% <0%> (-0.48%)	⬇️
planner/core/exhaust_physical_plans.go	93.81% <0%> (-0.32%)	⬇️
util/admin/admin.go	67.07% <0%> (-0.25%)	⬇️
sessionctx/variable/session.go	30.25% <0%> (-0.2%)	⬇️
... and 8 more

---

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[tiancaiamao]

I'll do a tiny refactor to remove the physicalID from three reader after this PR.

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[tiancaiamao]

/run-all-tests

[tiancaiamao]

/run-integration-compatibility-test

[tiancaiamao]

PTAL @lamxTyler

[alivxxx]

LGTM

[crazycs520]

@tiancaiamao Need cherry pick to v2.1.