The example is called "Privacy Preserving Search". If the server could see which row was accessed, that would not satisfy the privacy requirement. This implies that each query needs to process all the rows equally (and all in the same complicated encrypted way).
Provable privacy like here or in zero-knowledge proofs is generally extremely expensive because every possible execution path needs to be taken each invocation. This is multiplied by the overhead from each simple operation becoming a complex cryptographic one.
Provable privacy like here or in zero-knowledge proofs is generally extremely expensive because every possible execution path needs to be taken each invocation. This is multiplied by the overhead from each simple operation becoming a complex cryptographic one.