Support entityless features

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We have features that don’t have a corresponding entity and should just be joined based on the timestamp column.

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There are probably more efficient solutions than the one here but this one does a good job of preserving modularity such that it turns the entityless problem into a normal entity so all the existing logic/sql can run without needing to be adjusted to handle a special case. The code to turn the entityless case into a normal entity case can be quite self contained.

Feast supports an entity named “__entityless” which is internally defined. When an user registers a new featureview that involves entityless features, they pass an empty entity list. Empty list defaults to using the “__entityless” entity. Upon a join, feast looks at the entities involved with the features and if “__entityless” is present, the entity source data is appended with an “__entityless” column (SELECT *, 0 as __entityless from my_entity_table) and the feature data tables that use these entityless features are also appended with that constant column. __entityless is added to the join_key set. Everything else can proceed as normal.

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N/A

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[achals]

Assiging to @codyjlin

[mavysavydav]

i think you’ve already seen it, but we’ve start developing the design via this rfc - https://docs.google.com/document/d/1mOZ-a6Voq43JcsPu2BIaZEEa3FJ6RhyFxs2CZDbby-U/edit#heading=h.9gaqqtox9jg6