Now that you have seen what xlog is good for, it is necessary to discuss the concept of checkpoints. Let's assume we are inserting data just like we did earlier:

INSERT INTO foo VALUES ('abcdefg');

As you might expect, the ultimate goal of this INSERT statement is to write the new data to a table. In PostgreSQL, a table is always represented by a couple of datafiles consisting of 8k blocks (unless changed at compile time). The core question now is: is it desirable to write to the table directly? As discussed earlier in this chapter, writing to the table directly is not feasible anyway because in case of a crash, things would go south. Therefore, we go to the transaction log first. But this is not the end of the story as the next diagram shows:

Once the data has been sent to the transaction log, a copy of the future block on disk will be placed inside PostgreSQL shared buffers—data does not necessarily have to go to the table instantly; it is perfectly fine if the...