Mercy to Live

From Psalms 119.77, which I think is my second favorite Psalm after Psalm 139. It's packed so full of truth, and with my Bible reading plan (McCheyne) we go through it slowly enough to savor deeply.

What does the psalmist mean when he says, Let your mercy come to me that I might live? Is he speaking of that fact that it's by God's mercy that we live eternally? Or does he mean that we live our lives by God's mercy, as if to say, I will truly live to honor you by your mercy. I'm inclined to think its the latter because revelation at the time this was written saw death as a descent into the grave. I think he means to say, I only live properly if I live underneath your mercy, or compassion.