Log tables structure¶
The structure of log tables is directly derived from the structure of the related application tables. The log tables contain the same columns with the same type. But they also have some additional technical columns:
- emaj_verb : type of the SQL verb that generated the update (INS, UPD, DEL, TRU)
- emaj_tuple : row version (OLD for DEL, UPD and TRU ; NEW for INS and UPD ; empty string for TRUNCATE events)
- emaj_gid : log row identifier
- emaj_changed : log row insertion timestamp
- emaj_txid : transaction id (the PostgreSQL txid) that performed the update
- emaj_user : connection role that performed the update
When a TRUNCATE statement is executed for a table, each row of this table is recorded (with emaj_verb = TRU and emaj_tuple = OLD). A row is added, with emaj_verb = TRU, emaj_tuple = ‘’, the other columns being set to NULL. This row is used by the sql scripts generation.
Adding technical columns¶
It is possible to add one or several technical columns to enrich the traces. These columns value must be set as a default value (a DEFAULT clause) associated to a function (so that the log triggers are not impacted).
To add one or several technical columns, a parameter of key alter_log_table must be inserted into the emaj_param table. The associated text value must contain an ALTER TABLE clause. At the log table creation time, if the parameter exists, an ALTER TABLE statement with this parameter is executed.
For instance, one can add to log tables a column to record the value of the application_name connection field with:
INSERT INTO emaj.emaj_param (param_key, param_value_text) VALUES ('alter_log_table', 'ADD COLUMN extra_col_appname TEXT DEFAULT current_setting(''application_name'')');
Several ADD COLUMN directives may be concatenated, separated by a comma. For instance, to create columns recording the ip adress and port of the connected client:
INSERT INTO emaj.emaj_param (param_key, param_value_text) VALUES ('alter_log_table', 'ADD COLUMN emaj_user_ip INET DEFAULT inet_client_addr(), ADD COLUMN emaj_user_port INT DEFAULT inet_client_port()');
To change the structure of existing log tables once the alter_log_table parameter has been set, the tables groups must be dropped and then recreated.