Rails Migration and Dependencies on User Defined Tables/Views

Thinknear Engineering values reporting because it allows us to monitor how our mobile advertising platform behaves in its real time bidding environment. We also value business intelligence, because it gives Thinknear's operations and data science teams a tool to perform analytics and fine tune our marketing platform to ensure it meets our customers' needs and exceeds their expectations.

To this purpose, we are using AWS' Amazon Redshift as the main tool of our BI stack and we handle data loads and migrations through our Ruby on Rails applications. Being a BI tool, our redshift clusters need to accommodate tables and views created not only by our applications but by our operations and data science teams, as well. It is quite common for user defined tables and views to rely on application defined tables and views, which makes migrations a challenge.

In the following post, we present two SQL queries that are useful when trying to identify dependencies before running migrations.

Find locks held by running queries

Queries ran by users can hold locks on tables that the migration might operate on. If a user query holds a lock on a table that is bound to change by a migration, the migration will hang until the user query finishes (which in our case can sometimes be several hours). Empirically, we have found that querying the catalog view PG_LOCKS for finding existing locks tends to be more accurate than querying the STV_LOCKS table. Based on that, the following query returns all the locks on database objects (in any schema) in a redshift cluster.

SELECT getdate(),
FROM pg_catalog.pg_locks l
JOIN pg_catalog.pg_class c ON c.oid = l.relation
JOIN  pg_namespace n ON n.oid = c.relnamespace
JOIN pg_catalog.pg_stat_activity a ON a.procpid = l.pid;

In the query above:

  1. getdate() is the current timestamp. See GETDATE documentation
  2. c.relname is the name of the table or view the lock is held on.
  3. n.nspname is the name of the schema that the table or view belongs to.
  4. l.database is the current database id or a special value as described in PG_LOCKS table documentation.
  5. l.transaction is the transaction the query belongs to.
  6. l.pid is the pid of the query that obtained the lock.
  7. a.usename is the name of the user that issued the query that holds the lock.
  8. l.mode is the lock type (e.g. AccessShareLock).
  9. l.granted is true or false if the lock was granted or not.

This query is slightly better than the one provided in the STV_LOCKS example since it will give the username and table or view name out of the box. As a result, the engineer who runs the migration knows which table or view might be an impediment and they would also know who to talk to, before running the migration.

Find dependencies for a specific column

One type of potential dependency between user defined objects and application defined ones could be a foreign keyconstraint. This type of constraint can be found using the query mentioned here. However, there are different types of dependencies apart from foreign key constraints.

An example of such direct dependency is a user defined view that is built up by columns of application defined table(s). Imagine there is a table called orders that is partitioned monthly in the redshift database. Now, let us assume that the operations team wants to create views that gather all orders to produce quarterly reports. As a result, one of these quarterly views might be defined by the operations team like this:

CREATE VIEW orders_2015_Q1 AS 
    SELECT column_name1, column_name2... from orders_201501 UNION ALL
    SELECT column_name1, column_name2... from orders_201502 UNION ALL
    SELECT column_name1, column_name2... from orders_201503 UNION ALL
    SELECT column_name1, column_name2... from orders_201504;

This view does not have a foreign key constraint to any of the four partitions of the orders table it uses. However, it has a dependency on the columns of those partitions. If a migration wants to drop or alter the type of any of columnscolumn_name1,column_name2..., the migration will fail. In the case of a drop column, a cascade clause can be used to drop the column to all dependent objects. Since the tables or views that reference the altered columns might be user defined, we need to ensure it is safe to use the cascade clause before running the migration.

The following query can be used to spot dependencies like the one explained above:

SELECT distinct dependee.relname   
FROM pg_depend   
JOIN pg_rewrite ON pg_depend.objid = pg_rewrite.oid   
JOIN pg_class as dependee ON pg_rewrite.ev_class = dependee.oid   
JOIN pg_class as dependent ON pg_depend.refobjid = dependent.oid   
JOIN pg_attribute ON pg_depend.refobjid = pg_attribute.attrelid   
AND pg_depend.refobjsubid = pg_attribute.attnum   
WHERE dependent.relname = 'orders_201503'  
AND pg_attribute.attnum > 0   
AND pg_attribute.attname = 'column_name1';   

The query above will find any database objects (tables, views) that use column column_name1 from table orders_201503.

Alternatively to using a cascade clause, one might want to drop the orders_2015_Q1 view and restore it after the migration has finished. The definition of a view can be found before the migration:

SELECT definition FROM pg_views WHERE viewname='orders_2015_Q1';

After the migration, the view can be restored using the new types of columns or omitting columns that were dropped.

Handle everything in the application

Obviously, the best approach is to strive for having only application code handle the schema definition in your BI database. If operations and data science are heavily using a view or table created outside of the application, then that view or table needs to be part of the application defined schema. However, there are cases where that is not efficient, since SQL proficient data scientists can quickly write SQL code to perform their analysis without having to request the product development team for a migration. In the end, it's a trade-off that the engineering team needs to measure, but if the schema can be manipulated by both users and the application, then the queries presented above can help identify potential migration problems in advance.

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