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feat: improving docs (substrait-io#283)
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@curino
curino committed Aug 9, 2022
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Expand Up @@ -312,14 +312,14 @@ If at least one grouping expression is present, the aggregation is allowed to no

## Reference Operator

The reference operator is used to construct DAGs of operations. In a Plan we can have multiple Rel representing various
computations with potentially multiple outputs. The ReferenceRel is used to express the fact that multiple Rel might be
The reference operator is used to construct DAGs of operations. In a `Plan` we can have multiple Rel representing various
computations with potentially multiple outputs. The `ReferenceRel` is used to express the fact that multiple `Rel` might be
sharing subtrees of computation. This can be used to express arbitrary DAGs as well as represent multi-query optimizations.

As a concrete example think about two queries SELECT * FROM A JOIN B JOIN C and SELECT * FROM A JOIN B JOIN D,
We could use the ReferenceRel to highlight the shared A JOIN B between the two queries, by creating a plan with 3 Rel.
One expressing <A JOIN B> (in position 0 in the plan), one using reference as follows: <ReferenceRel(0) JOIN C> and a third one
doing <ReferenceRel(0) JOIN D>. This allows to avoid the redundancy of A JOIN B.
As a concrete example think about two queries `SELECT * FROM A JOIN B JOIN C` and `SELECT * FROM A JOIN B JOIN D`,
We could use the `ReferenceRel` to highlight the shared `A JOIN B` between the two queries, by creating a plan with 3 `Rel`.
One expressing `A JOIN B` (in position 0 in the plan), one using reference as follows: `ReferenceRel(0) JOIN C` and a third one
doing `ReferenceRel(0) JOIN D`. This allows to avoid the redundancy of `A JOIN B`.

| Signature | Value |
| -------------------- | --------------- |
Expand All @@ -331,9 +331,9 @@ doing <ReferenceRel(0) JOIN D>. This allows to avoid the redundancy of A JOIN B.

### Reference Properties

| Property | Description | Required |
|-----------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| --------------------------- |
| Referred Rel | A positional reference to a Rel defined within the same Plan. | Required |
| Property | Description | Required |
|-----------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| --------------------------- |
| Referred Rel | A positional reference to a `Rel` defined within the same `Plan`. | Required |

=== "ReferenceRel Message"

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