Single match, no index, no index. The retrieval speed can only be accelerated by concurrency, and the fastest sentence should be: select/*+full (a) parallel (a, 10) */* from a where b = c；; There are two reasons: when using 2: full table scanning, oracle will read multiple blocks at a time to speed up parallelism, and specify 10 threads for retrieval. Of course, if allowed, you can specify 20. Preferably equal to the number of CPUs. However, it is recommended to establish an index on column B, which is the best method.