like give query: select name employee id=23102 , sir_name="raj"; want know using algorithm search happen? assuming have indexed id field , unique. algorithm binary search (there optimizations , improvements, below general theory behind it). lets have following ordered list of numbers: 1,45,87,111,405,568,620,945,1100,5000,5102,5238,5349,5520 say want search number 5000, there 2 ways. scan entire list, in case have check 10 numbers (count start until reach 5000). binary -> here steps: 2a. go middle number (620), since 5000 bigger that-> 2b. same on numbers 945-5520, median 5102 since 5000 smaller that-> 2c. go median of part 945-5102, 1100 since lower 5000 go part between 1100-5102 2d. found it! that 4 operation against 10, so, binary search complexity grow @ same rate full scan when in binary search data grows exponentially