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https://github.com/FerretDB/documentdb.git
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Vinod Sridharan
84b999ae88
### Does this PR have any customer impact?
Yes
### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc)
Feature
### Does it involve schema level changes? (Table, Column, Index, UDF, etc level changes)
No
### Are you introducing any new config? If yes, do you have tests with and without them being set?
Yes
### ChangeLog (Refer [Template](../oss/CHANGELOG.md))
### Description
Improve pushdown of $elemMatch for composite indexes.
When there's an elemMatch of the form `{ "a": { "$elemMatch": { "$gt": 10, "$lt": 15 }}`
This can scan the index only from 10 -> 15
However given the multi-key index requirement we were doing 2 scans:
- One from 10 to infinity and the other from -infinity to 15.
Optimize this path for $elemMatch.
----
#### AI description (iteration 1)
#### PR Classification
This PR introduces a new feature that optimizes the pushdown of `$elemMatch` queries for composite indexes.
#### PR Summary
The pull request implements optimized handling of `$elemMatch` predicates on composite indexes by converting them into specialized index pushdown expressions and updating the corresponding test expectations.
- **`oss/pg_documentdb/src/opclass/bson_gin_composite_core.c` & `index_support.c`**: Added logic to detect `$elemMatch` in range queries and convert them into optimized pushdown expressions (via functions like `GetElemMatchIndexPushdownOperator`).
- **Configuration Changes**: Introduced a new GUC (`documentdb.useNewElemMatchIndexOperatorOnPushdown`) in `pg_documentdb/src/configs/feature_flag_configs.c` to control the new behavior.
- **Test Updates**: Modified expected explain outputs in both `/oss/internal/pg_documentdb_distributed/src/test/regress/` and `/pgmongo/src/test/docdb_compat/expected/` to reflect the revised index condition format using `elemMatchIndexOp`.
<!-- GitOpsUserAgent=GitOps.Apps.Server.pullrequestcopilot -->
Signed-off-by: Shuai Tian <shuaitian@microsoft.com>