While my clients query with compound field like

{a: x, b: y}

, my mongodb server automatically started to build compound index

{a: 1, b: 1}

at the same. Then my server became unresponsive for a while, which made my clients get null result when querying.
I checked the mongo log and see the following lines: "build index db.col

{ a: 1, b: 1 }

", happen very frequently; although I don't define that compound index.
I double checked the col index schema, and didn't find the declaration about the compound index

{a: 1, b: 1}

.
Please enlighten me, I had been checking all mongodb documents regarding to this strange behavior but found nothing. It does cause problem.