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feat: support with ties in FetchRel #797

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d79e6c3
feat: support with ties in FetchRel
yongchul Mar 20, 2025
9074ce4
split with tie and sorts such that effectively capturing (ORDER BY)? …
yongchul Mar 28, 2025
05ef587
Addressing PR feedback.
yongchul Mar 28, 2025
9581eab
Merge branch 'main' into fetch_sql_standard
yongchul May 24, 2025
da71b95
Merge branch 'main' into fetch_sql_standard
yongchul May 29, 2025
a9d3bff
Merge branch 'main' into fetch_sql_standard
yongchul Jun 4, 2025
972baa0
Merge branch 'main' into fetch_sql_standard
yongchul Jul 2, 2025
94cf1c6
Merge branch 'main' into fetch_sql_standard
yongchul Jul 29, 2025
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13 changes: 13 additions & 0 deletions proto/substrait/algebra.proto
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Expand Up @@ -319,6 +319,19 @@ message FetchRel {
// Recommended type for count is int64.
Expression count_expr = 6;
}

// List of fields to sort by to retrieve the number of records specified by `count_mode`.
// At least one filed MUST be specified when `with_ties` is true.
//
// Note: the output records are in the order of `sorts` if at least one sort field is specified.
// Otherwise, the input orderedness is assumed.
repeated SortField sorts = 7;
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@yongchul yongchul Jun 4, 2025

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During sync up, @jacques-n asked what is the minimal thing need to be done to support the scenario. The minimal is to specify list of column comparisons the same way as the backing Sort. In the Fetch requirement is weaker than Sort because we do not care the order across columns as well as directions but we do need to align how we calculate the equality of a column.

This (comparison, equality) happen two places in Substrait today: SortField and ComparisonJoinKey but they are separate.

Perhaps, we can do is

message EqualField {
  Expression.FieldReference field = 1;
  ComparisonJoinKey.ComparisonType comparison = 2;
}

message FetchRel {
  ..
  repeated EqualField tie_breakers = 7;
  bool with_ties = 8; // optional. maybe implicit based on non-empty `tie_breakers` field.
  ..
}

Then to implement with_ties, the producer must put the appropriate Sort below and setting up the tie breaking fields consistent to the Sort. I see a value in this approach (keeping a naive implementation of fetch simple) but not quite sure whether this is simpler than the proposed change, especially in the sense that a producer can concisely expression what needs to be done, and consumer has an option to implement or execute how.


// Whether to yields 'tie' records of the last record specified by `count_mode`.
// Tie is determined by the `sorts` order (i.e., `sorts` field values are the same if records are 'tie').
// If set to true, `sorts` MUST specify at least one sort field.
bool with_ties = 8;

substrait.extensions.AdvancedExtension advanced_extension = 10;
}

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8 changes: 5 additions & 3 deletions site/docs/relations/logical_relations.md
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Expand Up @@ -341,22 +341,24 @@ The output type is as follows for the various operations

## Fetch Operation

The fetch operation eliminates records outside a desired window. Typically corresponds to a fetch/offset SQL clause. Will only returns records between the start offset and the end offset.
The fetch operation eliminates records outside a desired window. Typically corresponds to an fetch/offset SQL clause. Will only returns records between the start offset and the end offset.

| Signature | Value |
| -------------------- | --------------------------------------- |
| Inputs | 1 |
| Outputs | 1 |
| Property Maintenance | Maintains distribution and orderedness. |
| Property Maintenance | Maintains input distribution. Orderedness is `Sort Fields` if specified. Otherwise, input orderedness. |
| Direct Output Order | Unchanged from input. |

### Fetch Properties

| Property | Description | Required |
| ----------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------- |
| Input | A relational input, typically with a desired orderedness property. | Required |
| Input | A relational input. | Required |
| Offset Expression | An expression which evaluates to a non-negative integer or null (recommended type is `i64`). Declares the offset for retrieval of records. An expression evaluating to null is treated as 0. | Optional, defaults to a 0 literal. |
| Count Expression | An expression which evaluates to a non-negative integer or null (recommended type is `i64`). Declares the number of records that should be returned. An expression evaluating to null indicates that all records should be returned. | Optional, defaults to a null literal. |
| Sort Fields | List of one or more sort fields to define a desired window. Requires at least one sort field if 'With Ties' is true. | Optional, defaults to empty sort fields. |
| With Ties | Whether to return tie rows of the last row specified by 'Count Expression'. If false, at most 'Count Expression' records are returned. If true, it may yield more recrods than 'Count Expression'. | Optional, defaults to false. |

=== "FetchRel Message"

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