I might be totally wrong...and it probably depends upon how you handle missing keys...if id and key_n form the compound unique index...then this might be useful (is in my case ;-)

{id} {key_1} = {value} 
{id} {""}    = {value} 
{id} {key_2} = {value} 
{""} {key_1} = {value} 
{""} {key_2} = {value}
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