Understanding noteworthy error scenarios
After going through the basic guidelines to hunt down the most common issues that you will face in your database, the upcoming sections will discuss some of the most common error scenarios that occur in the PostgreSQL world.
Facing clog corruption
PostgreSQL has a thing called the commit log (now called pg_xact; it was formally known as pg_clog). This tracks the state of every transaction on the system and helps PostgreSQL determine whether a row can be seen. In general, a transaction can be in four states:
#define TRANSACTION_STATUS_IN_PROGRESSÂ Â Â Â 0x00 #define TRANSACTION_STATUS_COMMITTEDÂ Â Â Â Â Â 0x01 #define TRANSACTION_STATUS_ABORTEDÂ Â Â Â Â Â Â Â 0x02 #define TRANSACTION_STATUS_SUB_COMMITTEDÂ Â 0x03
The clog has a separate directory in the PostgreSQL database instance (pg_xact).
In the past, people have reported something called clog corruption...
