In order to be able to perform a rollback operation without having previously kept a physical image of the PostgreSQL instance’s files, all updates applied on application tables must be recorded, so that they can be cancelled.
With E-Maj, this updates recording takes the following form.
Logged SQL statements¶
The recorded update operations concerns the following SQL verbs:
- rows insertions:
- INSERT, either elementary (INSERT … VALUES) or set oriented (INSERT … SELECT)
- COPY … FROM
- rows updates:
- rows deletions:
- tables truncations
For statements that process several rows, each creation, update or deletion is individually recorded. For instance, if a DELETE FROM <table> is performed against a table having 1 million rows, 1 million row deletion events are recorded.
The case of TRUNCATE SQL verbs is specific. As no FOR EACH ROW trigger can be fired for this verb, the consequences of a TRUNCATE cannot be cancelled by E-Maj. Therefore, its execution is forbidden for ROLLBACKABLE tables groups in LOGGING state. In contrast, TRUNCATE is always permitted for AUDIT_ONLY tables groups. In such a case, only its execution is recorded.
For each application table, the following objects are created:
- a dedicated log table, containing data corresponding to the updates applied on the application table,
- a trigger and a specific function, that, for each row creation (INSERT, COPY), change (UPDATE) or suppression (DELETE), record into the log table all data needed to potentially cancel later this elementary action,
- another trigger, that either blocks any execution of a TRUNCATE SQL verb for ROLLBACKABLE tables groups or records the execution of a TRUNCATE SQL verb for AUDIT_ONLY tables groups,
- a sequence used to quickly count the number of updates recorded in log tables between 2 marks.
A log table has the same structure as its corresponding application table. However, it contains some additional technical columns.
To let E-Maj work, some other technical objects are also created at extension installation time:
- 15 tables,
- 8 composite and 3 enum types,
- 1 view,
- 2 triggers,
- more than 120 functions, about 50 of them being directly callable by users,
- 1 sequence named emaj_global_seq used to assign to every update recorded in any log table of the database a unique identifier with an increasing value over time,
- 1 specific schema, named emaj, that contains all these relational objects,
- 2 roles acting as groups (NOLOGIN): emaj_adm to manage E-Maj components, and emaj_viewer to only look at E-Maj components
- 3 event triggers.
Some technical tables, whose structure is interesting to know, are described in details: emaj_group_def, emaj_param and emaj_hist.
Norm for E-Maj objects naming¶
All objects associated to application tables have names built by default with the name of their related table and schema. More precisely, for an application table in a given schema:
- the name of the log table is:
- the name of the log function is:
- the name of the sequence associated to the log table is:
It is also possible to define for each application table the prefix of the associated E-Maj objects name. This allows to manage tables with very long names.
A log table name may contain a suffix like “_1”, “_2”, etc. In such a case, it deals with an old log table that has been renamed by an emaj_alter_group operation.
Other E-Maj function names are also normalised:
- function names that begin with emaj_ are functions that are callable by users,
- function names that begin with _ are internal functions that should not be called directly.
Triggers created on application tables have the same name:
- emaj_log_trg for the log triggers,
- emaj_trunc_trg for the triggers that manage TRUNCATE verbs.
The name of event triggers starts with emaj_ and ends with _trg.
All technical objects created at E-Maj installation are located into the schema named emaj.
By default, all objects linked to a tables group are created in the main schema emaj. But it is possible to locate these objects in one or several secondary schemas. Secondary schemas’ names start with emaj, only their suffix being parametrized in tables groups definition.
Only one technical object is not located into the emaj schema: the event trigger emaj_protection_trg belongs to the public schema.
When the extension is installed, the E-Maj technical tables are stored into the default tablespace set at instance or database level or explicitely set for the current session.
The same rule applies for log tables and index. But using tables group parameters, it is also possible to store log tables and/or their index into specific tablespaces.