Speedup compile times for joins #3288
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`DefaultSelection` type of joins This commit introduces infrastructure that allows to skip type checking the `DefaultSelection` of joins. diesel-rs#3223 discovered that this is responsible for a substantial amount of compile time for larger joins/codebases. We do not need to check this constraint there as it is always true because each of the tables/sides of the join already have an valid `DefaultSelection`. Therefore the resulting `DefaultSelection` must be valid for the join too (if there is no bug in diesel itself).
it doesn't compile as-is.
The reason for this is that with this implem,
```rust
let q = schema::some_table_1::table
.inner_join(schema::some_table_2::table)
.inner_join(schema::some_table_3::table.inner_join(schema::some_table_4::table));
```
won't compile, because of this bound:
```
impl<Left, Right> QuerySource for Join<Left, Right, Inner>
where
Left: AppendSelection<Right::DefaultSelection>,
```
and we don't have `Left = Join<LeftLeft, LeftRight, Inner>: AppendSelection<SkipSelectableExpressionWrapper<_>>`
because we don't have `SkipSelectableExpressionWrapper<_>:
TupleAppend<SkipSelectableExpressionWrapper<_>>`
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Mingun
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Aug 23, 2022
diesel/src/query_source/joins.rs
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| // trait implementations to the inner type | ||
| // | ||
| // See https://github.com/diesel-rs/diesel/issues/3223 for details | ||
| type DefaultSelection = self::private::SkipSelectableExpressionWrapper<Left::Output>; |
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Interesting... How did you trick the compiler into disabling check? For me, this looks like an insoluble task
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The relevant line is here: https://github.com/diesel-rs/diesel/pull/3288/files#diff-d0e8a449e498610baa9a5f3086200726d02bd5e1612f5f1d5587c4418baddf0eR482
This impl does not check that the trait holds true for the inner type, so it effectively skips the recursive trait bound check on large tuples, which is the underlying problem.
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You mean, that the most important thing there that T is not bounded, so the compiler does not do expensive recursive checks as it did before?
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Yes that's the main optimization.
Ten0
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Aug 24, 2022
Co-authored-by: Ten0 <9094255+Ten0@users.noreply.github.com>
…BoundCheckWrapper` as suggested in review
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Addresses #3223