Database Star Schemas and Snowflake Schemas

Schema Confusion

A lot of people very regularly work with databases (even high end ones), but get thrown by terms like star-schema, snowflake-schema, etc. due to lack of formal training or working with data warehousing technologies.

These same people will often be perfectly comfortable with indexing, query optimization, foreign keys, concepts of de-normalization and normal forms, etc.

I personally started working with the actual “Snowflake” database recently and had to review what a snowflake shema was when I started looking at it.

Useful Articles

I found an interesting article on Star schemas vs Snowflake schemas pretty quickly, and back tracked it to precursor articles digging into the Star and Snowflake schemas respectively.  Here are each in case you want the original content; I’m just going to paraphrase it below to give people a quick overview and/or refresher.

Star Schema

A star schema just means that your main table has a primary key made out of multiple columns, each of which is a foreign key to a “dimension” table.  Then you have one or more “fact” columns in addition to the primary key.

The dimension columns will be all the relevant attributes you may want to aggregate and/or query the main table on.  For example, you might have a table for the date which breaks out the year, month, day, and day-of-week so they can be directly used.  You may then have another dimension table for the geographical region with columns for the continent, country, and city, for example, so you can aggregate on those.

Each dimension table is NOT de-normalized though.  So, if you have “New York City” as the city for 1 million rows, you are literally repeating that a million times.  This makes queries easy to write but has a penalty in terms of data storage (which can be bad if you’re, say, in the cloud and paying more for more storage over time).

Snowflake Schema

Plain and simple; a snowflake schema is a star schema where the dimension tables are normalized.  This means that, for example, the geographical region dimension table itself would actually be turned into 4 tables (kind of its own star schema).  You would have one table for the continent, one for the country, one for the city, and one main table for the combination of the 3 as a primary key.

This makes queries more complex and possibly a little slower, but it means we have complete normalization and are not wasting any data storage.  Also, if say, a city changed its name, we would have exactly one database cell to update where as in a star schema we would have to update potentially millions of rows with copies of that name.

Why the Names?

If you think of a “Star Schema”, picture a main table with, say, 5 extra dimension tables around it like the 5 points of a star.  Makes sense, right?

Now, for a snowflake, picture each point being 5 tables by itself… so each point is its own star.  This starts to branch out like a snowflake.  Just think of fractals if you don’t believe me :).



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