Logging sequential scans in PostgreSQL?
I don't know a way to log sequential scans only.  Just a small remark: seqscans are not evil, you will get them (especially for smaller tables) all the time.

On the other hand, you can definitely log everything, if you want, and there are multiple ways.

The easiest is probably 

[ALTER DATABASE yourdb ] SET log_statement TO 'all';


This will log every query that passes the parser:


  Note: Statements that contain simple syntax errors are not logged even by the log_statement = all setting, because the log message is emitted only after basic parsing has been done to determine the statement type. In the case of extended query protocol, this setting likewise does not log statements that fail before the Execute phase (i.e., during parse analysis or planning). Set log_min_error_statement to ERROR (or lower) to log such statements.


Another insteresting way of achieving it is setting log_min_duration_statement to a (very) low value:

[ALTER DATABASE yourdb ] SET log_min_duration_statement TO 0; -- or somewhat higher


The difference between the two is that the latter will also register the actual parameters passed to functions.

Notes:


you need superuser rights for changing either of these settings
you can use both at the same time.  For details, please read the documentation linked above.