Jared Rhodes  

PostgreSQL Historical Log by Table

In my current project, there is a need for tracking data changes in the PostgreSQL tables. The end goal is, if a row changes, we copy the previous row before the change transaction completes and write it to a logging table. We will accomplish this with in the following steps:

  •  Creating a table LIKE our table that needs logging
  •  Create a function for our table
  •  Apply that function as a trigger

Table Like

First we need an example table to get started with. For a simple example, lets use a basic address table.

This basic table has enough constraints to make a decent example. We need to create a copy of this table, one where the columns are the same name and type, but without all of the constraints. Luckily for us, PostgreSQL provides a feature for just a situation. For this, we want to use the like_option for the CREATE TABLE statement. According the latest documentation (PostgreSQL 12) at the time of writing this post:

The LIKE clause specifies a table from which the new table automatically copies all column names, their data types, and their not-null constraints.

Unlike INHERITS, the new table and original table are completely decoupled after creation is complete. Changes to the original table will not be applied to the new table, and it is not possible to include data of the new table in scans of the original table.

Also unlike INHERITS, columns and constraints copied by LIKE are not merged with similarly named columns and constraints. If the same name is specified explicitly or in another LIKE clause, an error is signaled.

The optional like_option clauses specify which additional properties of the original table to copy. Specifying INCLUDING copies the property, specifying EXCLUDING omits the property. EXCLUDING is the default. If multiple specifications are made for the same kind of object, the last one is used.

We will want to exclude all constraints so that when our trigger fires, it can write any data to the columns without worrying if those columns are valid. The resulting table definition looks like the following:

Logging Function

Next, there needs to be a trigger that logs the data. To create a new trigger in PostgreSQL, you follow these steps:

    • First, create a trigger function using CREATE FUNCTION statement.
    • Second, bind the trigger function to a table by using CREATE TRIGGER statement.

A trigger function is similar to an ordinary function. However, a trigger function does not take any argument and has a return value with the type of trigger. Inside this trigger function, insert the old data into the logging table. This makes the trigger function as follows:

TG_OP is Data type text; a string of INSERT, UPDATE, DELETE, or TRUNCATE telling for which operation the trigger was fired.

Implementing the Trigger

As we said earlier, Second, bind the trigger function to a table by using CREATE TRIGGER statement. This part is fairly easy.

Now, whenever you insert, update, or delete a record in the address table, the operation is logged in the logging address table.

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