Database design - boolean attribute or new table

Question

Let\'s say I have a database table that I want to filter based on a boolean attribute (for example, \"flagged\" attribute). Is it better to just add a \"flagged\" attribute

Answer 1

If that is all you need¹, then just add the simple field.

But, you'll have to be careful about how you index it. Unless the values are seriously skewed², you'll end-up with an index with a horrible selectivity and clustering factor³, and you'll actually be better off doing full table scans⁴.

If you filter by the flag in conjunction with other fields, make a composite index, which will then have a much better chance of being decently selective.

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¹ I.e. you don't need additional data that somehow "describes" or "augments" each of the two possible Boolean values.

² One value is in deep minority (the number of rows containing false is much smaller than the number of rows containing true, or vice verse), and you happen to filter on just that value.

³ The link is for Oracle, but the general principle applies to all DBMSes.

⁴ Which a decent DBMS will do for you automatically, even in the presence of the index. A hypothetical "stupid" DBMS would just blindly use the index and perform even worse than the full table scan.