JasonLi-cn commented on PR #12184:
URL: https://github.com/apache/datafusion/pull/12184#issuecomment-2311677644
> > > The plan looks like aliasing to the same name after
`optimize_projections`
> > > ```
> > > Projection: test.a AS test.a
> > > TableScan: test projection=[a]
> > > ```
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > I wonder could we even remove the alias here, since the name is the
same
> >
> >
> > Removing the Alias will cause a schema change. 🤔
>
> That is why I think the root cause might be the incorrect schema we have.
>
> `SELECT "test.a" FROM (SELECT a AS "test.a" FROM test)`
>
> The expected schema should be something like.
>
> ```
> DFSchema { inner: Schema { fields: [Field { name: "test.a", data_type:
Int32, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }],
metadata: {} }, field_qualifiers: [Some(Bare { table: "test" })],
functional_dependencies: FunctionalDependencies { deps: [] } }.
> ```
>
> a is aliased to "test.a". The column should be `test."test.a"`
>
> Upd:
>
> ```
> query I
> SELECT a FROM (SELECT a FROM test)
> ----
> 1
> ```
>
> This is the schema for the query without alias
>
> ```
> schema: DFSchema { inner: Schema { fields: [Field { name: "a", data_type:
Int32, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }],
metadata: {} }, field_qualifiers: [Some(Bare { table: "test" })],
functional_dependencies: FunctionalDependencies { deps: [] } }
> ```
>
> ```
> query I
> SELECT b FROM (SELECT a as b FROM test)
> ----
> 1
> ```
>
> This is the schema with alias
>
> ```
> schema: DFSchema { inner: Schema { fields: [Field { name: "b", data_type:
Int32, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }],
metadata: {} }, field_qualifiers: [None], functional_dependencies:
FunctionalDependencies { deps: [] } }
> ```
>
> `field_qualifiers` is empty for alias case. Does that mean we didn't
preserve `field_qualifiers` for alias. 🤔
>
> ```
> field_qualifiers: [Some(Bare { table: "test" })]
> ```
Yes. Possibly because the `b` field itself is not part of the `test` table?
🤔
If we make the following changes to the SQL, the `b` field has
field_qualifiers: `[Some(Bare { table: "t0" })]`
```
query I
SELECT b FROM (SELECT a as b FROM test) t0
----
1
```
```
schema: DFSchema { inner: Schema { fields: [Field { name: "b", data_type:
Int32, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }],
metadata: {} }, field_qualifiers: [Some(Bare { table: "t0" })],
functional_dependencies: FunctionalDependencies { deps: [] } }
```
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