1. Binary search.
This requires the record yo be organised according to the key field - i.e. when you add a new record you need to place it at the appropriate position in the file and then move all subsequent records down one place down to accommodate it. If the key number of the record you are looking for is smaller than this, then you read the key number of the record halfway between the current record and the beginning of the file (i.e. one quarter way down the file). If your required record has a higher key number than the record at the quarter-point, then you read the record halfway between this and the last record known to be higher (i.e. the one at the mid-point). The process is repeated iterated iteratively, each time reducing the search area by half, until you eventually zoom in on the required record.