Every modern database system provides functionality to make sure that the system can survive a crash in case something goes wrong or somebody pulls the plug. This is true for filesystems and database systems alike.
PostgreSQL also provides a means to ensure that a crash cannot harm the data's integrity or the data itself. It is guaranteed that if the power cuts out, the system will always be able to come back on again and do its job.
The means of providing this kind of security is achieved by the Write Ahead Log (WAL), or xlog. The idea is to not write into the data file directly, but instead to write to the log first. Why is this important? Imagine that we are writing some data, as follows:
INSERT INTO data ... VALUES ('12345678');
Let's assume that this data was written directly to the data file. If the operation fails midway, the data file would be corrupted. It might contain half-written rows, columns without index pointers, missing...
