Executing Multiple Dynamic SQL Strings in PostgreSQL Using the DO Statement

Executing Dynamic SQL Strings

Overview

In this article, we will explore how to execute multiple SQL strings created dynamically using PostgreSQL. We will cover the various approaches and techniques used in the solution.

Introduction to Dynamic SQL

Dynamic SQL is a feature of most programming languages that allows you to generate SQL commands at runtime based on user input or other dynamic data. In PostgreSQL, dynamic SQL can be used with the EXECUTE statement, which allows you to execute a dynamically generated SQL command.

However, when it comes to executing multiple SQL strings created dynamically, things can get a bit more complex. This is where the DO statement comes into play.

The Problem

The problem with executing dynamic SQL strings is that each string needs to be executed separately, which can lead to issues if not handled properly.

SELECT                          
    'CREATE TABLE IF NOT EXISTS '
    || tab_name || '_audit(timestamp TIMESTAMPTZ NOT NULL, entity JSONB NOT NULL);'
FROM (
    SELECT                                                                     
        quote_ident(table_schema) || '.' || quote_ident(table_name) as tab_name
    FROM                         
        information_schema.tables
    WHERE                                                       
        table_schema NOT IN ('pg_catalog', 'information_schema')
        AND table_schema NOT LIKE 'pg_toast%'
) tablist;

This generates multiple rows of SQL code, which can be executed separately using the EXECUTE statement.

DO
$$
DECLARE
   _sql text;
BEGIN
   FOR _sql IN
      SELECT format('CREATE TABLE IF NOT EXISTS %I.%I(timestamp timestamptz NOT NULL, entity jsonb NOT NULL)'
                  , schemaname
                  , tablename || '_audit')
      FROM   pg_catalog.pg_tables  -- only tables and partitioned tables, no toast tables
      WHERE  schemaname NOT IN ('pg_catalog', 'information_schema')
   LOOP
      RAISE NOTICE '%', _sql;
      -- EXECUTE _sql;
   END LOOP;
END
$$;

Solution

The solution uses a DO statement to execute the dynamically generated SQL strings. The DO statement is a PostgreSQL feature that allows you to define a block of code as a stored procedure, which can then be executed like any other procedure.

In this case, we use a FOR loop to iterate over each row of dynamic SQL code generated from the information_schema.tables system view. We use the format() function to generate the dynamic SQL code for each table.

The _sql variable stores the current SQL string being processed, which is then raised as a notice using the RAISE NOTICE statement. This allows us to inspect the payload before executing it.

To execute the SQL strings, we uncomment the line that calls the EXECUTE statement with the stored SQL string.

Code Explanation

The code uses several PostgreSQL-specific features and functions:

quote_ident(table_name) is used to quote the table name properly. This ensures that the table name is correctly formatted for use in SQL commands.
format('CREATE TABLE IF NOT EXISTS %I.%I(timestamp timestamptz NOT NULL, entity jsonb NOT NULL)', schemaname, tablename || '_audit') generates a dynamically formatted SQL string. The %I and %I placeholders are used to specify the schema name and table name respectively.
pg_catalog.pg_tables is used instead of information_schema.tables. This view provides more detailed information about the tables in the database and can be faster to query than the information_schema.tables view.

Best Practices

When working with dynamic SQL, it’s essential to follow best practices to ensure that your code is secure and reliable. Here are some tips:

Always validate user input before using it to generate dynamic SQL commands.
Use parameterized queries or prepared statements to avoid SQL injection attacks.
Test your dynamic SQL code thoroughly to ensure that it produces the expected results.

Conclusion

In this article, we explored how to execute multiple SQL strings created dynamically in PostgreSQL. We covered the various approaches and techniques used in the solution and provided a detailed explanation of the code used.

By following the best practices outlined in this article, you can create robust and secure dynamic SQL solutions for your PostgreSQL applications.

Additional Reading

For more information on dynamic SQL in PostgreSQL, see:

Next Steps

If you’re new to dynamic SQL in PostgreSQL, we recommend that you start by learning the basics of PostgreSQL and its SQL language. Once you have a solid understanding of the fundamentals, you can move on to more advanced topics like dynamic SQL.

In our next article, we’ll explore how to use dynamic SQL to create and manage database connections.

Last modified on 2024-06-04