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Type:
Improvement
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: Aggregation Framework
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None
Observed behavior: A $and/$or expression with a single constant is not optimized as well as a similar expression with two constants.
Expected behavior: An expression with one constant should be optimized the same way as a similar expression with two constants.
ExpressionAnd::optimize() and ExpressionOr::optimize() expect that any constant will appear in the last operand, but ExpressionNary::optimize() does not guarantee placement of a constant in the last operand if there was only one constant to begin with.
/*
If there's no more than one constant, then we can't do any
constant folding, so don't bother going any further.
*/
if (constCount <= 1)
return intrusive_ptr<Expression>(this);
Test:
c = db.c;
c.drop();
c.save( { x:true } );
// This expression is left as is.
printjson( c.runCommand( 'aggregate', { pipeline:[ { $project:{ a:{ $and:[ 0, '$x' ] } }\
} ], explain:true } ) );
// This expression is optimized to 'false', eliminating the need to read field 'x', due to the existence of two constant operands.
printjson( c.runCommand( 'aggregate', { pipeline:[ { $project:{ a:{ $and:[ 0, 1, '$x' ] \
} } } ], explain:true } ) );
// This expression is left as is.
printjson( c.runCommand( 'aggregate', { pipeline:[ { $project:{ a:{ $or:[ 1, '$x' ] } } \
} ], explain:true } ) );
// This expression is optimized to 'true', eliminating the need to read field 'x', due to the existence of two constant operands.
printjson( c.runCommand( 'aggregate', { pipeline:[ { $project:{ a:{ $or:[ 1, 0, '$x' ] }\
} } ], explain:true } ) );
- is related to
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- Closed
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