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  1. Core Server
  2. SERVER-83602

$or -> $in MatchExpression rewrite should not generate $or directly nested in another $or

    • Fully Compatible
    • v7.2, v7.0, v6.0, v5.0
    • QO 2024-02-05
    • 135

      As part of MatchExpression::optimize(), we have logic to try to rewrite an $or of equalities over the same path to an $in. This is advantageous, because it helps the downstream optimization code produce better plans. Here's an example of the rewrite:

      // Original match expression.
      {$or: [{name: "Don"}, {name: "Alice"}]}
      // This gets rewritten to the following.
      {name: {$in: ["Alice", "Don"]}}

      When not all of the clauses of the $or can get rewritten in this manner, the current implementation can output a match expression tree with an $or that is a direct child of another $or. Here's an example:

      // Original match expression.
      {$or: [{name: "Don"}, {name: "Alice"}, {age: 42}, {job: "Software Engineer"}]}
      // This gets rewritten to the following.
      {$or: [{name: {$in: ["Alice", "Don"]}}, {$or: [{age: 42}, {job: "Software Engineer"}]}]}

      As you can see, one of the direct children of the outer $or is another $or. There is a separate rewrite which happens as part of MatchExpression::optimize() which attempts to flatten such nested $or nodes. However, the $or -> $in rewrite happens afterwords. In the master branch, the $or-$or is subsequently simplified by the new boolean simplification module enabled in SERVER-81630, but in older branches the $or-$or is never simplified.

      I would argue that despite boolean simplification, we should modify the implementation of the $or -> $in rewrite to avoid constructing directly nested $or nodes. Continuing the example above, the output of the $or rewrite should be as follows:

      {$or: [{name: {$in: ["Alice", "Don"]}}, {age: 42}, {job: "Software Engineer"}]}

            david.storch@mongodb.com David Storch
            david.storch@mongodb.com David Storch
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