Mongodb performance - dont use $and when only a single condition is specified

Help me!
1

When a single condition is specified on a filter, the $and operator is still used.
As a result, a condition on a unique index like _id equality is not using an index lookup plan but rather might use an index scan.

A test that shows this behavior is this one.

I believe the fix can be rather easy - here when the formattedConditions is a single array element we could do:

this.formatAggregation = async (aggregator, formatedConditions) => {
+   if (formatedConditions.length === 1) return formatedConditions[0];
    const aggregatorOperator = this.formatAggregatorOperator(aggregator);
    return { [aggregatorOperator]: formatedConditions };
  };

Instead of today:

this.formatAggregation = async (aggregator, formatedConditions) => {
    const aggregatorOperator = this.formatAggregatorOperator(aggregator);
    return { [aggregatorOperator]: formatedConditions };
  };

Expected behavior

This could be a limitation of the mongodb planner - but in reality - the query generator from Liana could decide to not use $and and $or (condition aggregator operator) when a single filter condition is requested.

Actual behavior

$and is used even on a single condition filter.

Context

  • Package Version: “forest-express-mongoose”: “^6.1.0”
  • Database Version: Mongodb 3.6 (pretty sure this applies to 4 as well)
2

Hi @Yoad_Snapir :wave: thank you for your feedback!
It’s a good improvement, I’m transferring him to our team.

1 Like
3

Please note, I’m unable to reproduce this problem on mongodb 4.2.
(And this was not implemented in Liana, so it’s not that)