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## Description
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This PR allows DC agents to define custom aggregate functions for their scalar types.
### Related Issues
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GDC-189
### Solution and Design
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We added a new property `aggregate_functions` to the scalar types capabilities. This allows the agent author to specify a set of aggregate functions supported by each scalar type, along with the function's result type.
During GraphQL schema generation, the custom aggregate functions are available via a new method `getCustomAggregateOperators` on the `Backend` type class.
Custom functions are merged with the builtin aggregate functions when building GraphQL schemas for table aggregate fields and for `order_by` operators on array relations.
### Steps to test and verify
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• Codec tests for aggregate function capabilities have been added to the unit tests.
• Some custom aggregate operators have been added to the reference agent and are used in a new test in `api-tests`.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/6199
GitOrigin-RevId: e9c0d1617af93847c1493671fdbb794f573bde0c
## Description
This PR fixes hasura/graphql-engine#8345: when creating the final representation of a remote relationship to a remote schema (a `RemoteJoin`), we would mistakenly label ALL join fields in the selection set as being relevant to that one relationship: if there are more than one remote relationship to process in that selection set, that would be the union of all their join fields. The problem with this error is that, when processing remote relationships, we correctly ignore all the ones for which at least one join key is null. Consequently, this error would result in us ignoring remote relationships for which an _unrelated_ join key was null, resulting in that data missing in the final JSON result.
This PR simply ensures that the aggregation of fields that are passed to `createRemoteJoin` is pruned to only contain the fields relevant to the join being created. This is a very small change, and the bulk of this PR is the regression tests.
## Changelog
__Component__ : server
__Type__: bugfix
__Product__: community-edition
### Short Changelog
fix remote relationship to remote schema sometimes being erroneously null when multiple relationships are defined on the same table / graphql object ([#8345](https://github.com/hasura/graphql-engine/issues/8345))
### Long Changelog
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/6420
GitOrigin-RevId: eb54462724b007f80b674dcf234adf6d9cfaaf79
