Designing a Dimensional Schema for a hierarchy that can change over time
I’ve recently been reading about dimensional schemas, aka star schemas. The whole idea seems to be to optimize for fast queries that are also simple to write, at the expense of extra storage in the form of denormalization. In contrast to standard third normal form relational database schema, there is little to no emphasis on how complicated it will be to write data into the system. This trade-off makes sense, as star schemas are employed primarily for data warehouses, where the user will be a business intelligence analyst writing ad-hoc queries to pull analytics, not an ORM layer or another piece of pre-vetted code hitting previously defined and approved indexes.
The schema itself can be counter intuitive for engineers who have only worked on normal RDMS schemas. First off, data is broken down into fact tables, which are the events that the analytics are aggregating, and dimension tables, which are the attributes that the events will be rolled up by. For example, we are going to use the fact of social media posts as an example, while the dimensions will be the group that the posting user works at in their company, together with that groups place in the company org chart. To up the level of difficulty, we are also going to talk about how to model an org chart that is changing over time.
Here we have a somewhat complicated org chart, albeit a small one. Group 10 happens to roll up to both the State and Role hierarchies. This is known as a multi-hierarchy system. The company wants to be able to report on posts made by that group from two different perspectives, depending on the specific report they are trying to run.
Two particular challenges talked about in the excellent The Data Warehouse ETL Toolkit are modeling hierarchies (which are essentially trees, and also difficult to model in traditional RDMS systems) and slowly changing dimensions. For example, what happens if a group moves to a different spot in one of the hierarchies, but the company still wants to report on their posts according to the hierarchy they were actually in at the time? More on that later.
Test Schema and Data
Here is one model for the hierarchy information, aka the Region Bridge Table, or
test_region_bridge. For each of these tables, I will also give the SQL to create the table and insert the same test data.
|1||All Regions||2||State Hierarchy||1|
|1||All Regions||3||Role Hierarchy||1|
Notice that every node has a row for each of their children, whether they are direct children (depth 1) or further down. This is intended to make it possible to query for all groups under a certain level without explicitly listing all the children in a dependent sub-query. The parent and child names are given just to make it easier to see what your queries are doing.
Here is the
test_post table. Notice that it has a
group_id, but not a
Finally, here is the
test_group_region_bridge table, which joins
Given this schema, how would we write a query to answer the question How many posts were there in each region under the Role Hierarchy between Jun 1, 2014 and Jun 30, 2014?
Note that I’m including a
depth_from_parent = 1, otherwise my
JOIN would include a row for any regions that might be under either Sales or Eng, and count posts in the parent regions multiple times.
We could also ask How many posts per day were there under the State Hierarchy?
Which results in:
Also, Which posts were there under both Sales and MA?
This one is tricky, but shows the flexibility of SQL. It returns the correct single row.
Taking it up a notch
Now, I promised to address what happens when we also need to account for changes in the hierarchy over time. For example, what if group 11 moved from district 7 (Eng) to district 6 (Sales) on 2014-06-18, and then moved back to district 7 (Eng) on 2014-06-22? There should still be four posts, but now there were three in Sales and one in Eng.
In order to record the relevant dates, I’ll add
change_end dates to the table, as well as a
change_current to track whether this is the current state of the group. Here is our new
Notice that I’m using dates in the far past and far future to denote the beginning of time and the end of time, respectively. This is to make queries easier to write.
Now we can run the same region count query as before, but have it take into account that fact that the post has to have been made while the group was still in that region. You can also use the same schema to fall back to the previous behavior of looking at just the current state of the org chart with
change_current = 1.
Smoothing out the kinks
You may have noticed that these queries are all repeating the same
JOIN syntax. If you think this is going to be a common query pattern, you can create a view as a shortcut.
This will make your previous query much simpler.
This also abstracts the existence of the
change_* fields from the user. The hard part is communicating what the view is doing, so query authors know when to use it, and when NOT to use it. If you want to maximize the performance of the system at the cost of extra storage, you can make this a materialized view.