Remove $facet in top level group stages #449
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Awesome, thank you and thanks @asya999 |
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Can you add a test, particular one that would test something that breaks in the Django admin with QE (even though QE is not merged yet)?
django_mongodb_backend/compiler.py
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| pipeline.append({"$group": group}) | ||
| # It may be a bug, $$NOW has to be called to be reachable in the rest of the pipeline. |
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Please add link to the SERVER ticket and add an explanatory comment about the purpose of the $unionWith clause.
I think we can also mention it in the release notes as a performance improvement since the team said $facet prevents the use of optimizations.
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Sadly, this cannot be implemented as it is.
To handle the global aggregation and the default values (for example:
SELECT coalesce(avg(age), 35) FROM people WHERE age > 100;we need to wrap the aggregation so MongoDB always returns a single document.
This is done by wrapping the pipeline inside a lookup. Example:
db.z.aggregate([
{ $collStats: {} },
{ $lookup: { from: "z", as: "wrap", pipeline: [...pipeline stages] } },
{ $replaceWith: {"$cond": [{"$eq": ["$wrap", []]}, {}, {$first: "$wrap"}]}}
])
It’s not very intuitive use of $collStats, but it works 😄. It replaces the need for $unionWith.
$collStats is used as a pivot so we can apply the required operator after the group result.
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Can you provide, in this PR, an example of what the removal of $facet makes the new query signatures look like? |
WaVEV
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Generated query:
[{'$collStats': {}}, {'$lookup': {'as': 'wrapped', 'from': 'aggregation_author', 'pipeline': [{'$group': {'_id': None, 'count': {'$sum': {'$cond': {'else': 1, 'if': {'$in': [{'$type': '$_id'}, ['missing', 'null']]}, 'then': None}}}}}]}}, {'$replaceWith': {'$cond': [{'$eq': ['$wrapped', []]}, {}, {'$first': '$wrapped'}]}}, {'$project': {'count': {'$ifNull': ['$count', {'$literal': 0}]}}}]