executor: support "show columns from" for view

[AndrewDi]

What problem does this PR solve?

Modify DESCRIBE TABLE to describe view columns

What is changed and how it works?

ref proposal Proposal: Implement View

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
  Export function func (b *PlanBuilder) BuildDataSourceFromView(dbName model.CIStr, tableInfo *model.TableInfo)

Side effects
none

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This change is 

[AndrewDi]

@XuHuaiyu @winkyao PTAL

[XuHuaiyu]

Any test for this?

[AndrewDi]

@XuHuaiyu show_test.go do not contain any DESCRIBE TBALE test, so I didn't add test either.

[XuHuaiyu]

We'd better add some test cases for it.

[alivxxx]

LGTM

[AndrewDi]

/run-all-tests

[AndrewDi]

@XuHuaiyu PTAL

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@081a2c5). Click here to learn what that means.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##             master    #8863   +/-   ##
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  Coverage          ?   67.54%           
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  Files             ?      363           
  Lines             ?    75111           
  Branches          ?        0           
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  Hits              ?    50731           
  Misses            ?    19907           
  Partials          ?     4473

Impacted Files	Coverage Δ
planner/core/logical_plan_builder.go	74.02% <100%> (ø)
executor/show.go	41.47% <80%> (ø)

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[zz-jason]

We can't get the column type from the columns returned by tb.Cols()?

[AndrewDi]

Yes, view's column tableinfo only store column name, so we need to extract sql's schema.column to get column type.

[zz-jason]

Can we fill the column type info when this table info is generated?

[AndrewDi]

I don't think so, because undertable's column type and column length can be change.

[zz-jason]

MySQL(root@localhost:test) > create table t(a bigint, b bigint);
Query OK, 0 rows affected (0.04 sec)

MySQL(root@localhost:test) > create view v as select * from t;
Query OK, 0 rows affected (0.07 sec)

MySQL(root@localhost:test) > show columns from v;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| a     | bigint(20) | YES  |     | NULL    |       |
| b     | bigint(20) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

MySQL(root@localhost:test) > drop table t;
Query OK, 0 rows affected (0.03 sec)

MySQL(root@localhost:test) > create table t(a double, b double, c double);
Query OK, 0 rows affected (0.07 sec)

MySQL(root@localhost:test) > show columns from v;
+-------+--------+------+-----+---------+-------+
| Field | Type   | Null | Key | Default | Extra |
+-------+--------+------+-----+---------+-------+
| a     | double | YES  |     | NULL    |       |
| b     | double | YES  |     | NULL    |       |
+-------+--------+------+-----+---------+-------+
2 rows in set (0.01 sec)

Got it. Confirmed in MySQL.

[zz-jason]

@AndrewDi Could you add a comment to specify why we need to build logical plan on the view to get the column types?

[AndrewDi]

Done

[zz-jason]

LGTM

[zz-jason]

/run-all-tests