Query JOIN or not? (optimization)

Question

i was wondering if to use 2 tables is better then using 1 single table .

Scenario:

i have a simple user table and a simple user_details table. i can JOIN tables and select both records.

But i was wondering if to merge 2 table into 1 single table.

what if i have 2milions users records in both tables?

in terms of speed and exec time is better to have a single table when selecting records?

Answer 1

You should easily be able to make either scenario perform well with proper indexing. Two million rows is not that many for any modern RDBMS.

However, one table is a better design if rows in the two tables represent the same logical entity. If the user table has a 1:1 relationship with the user_detail table, you should (probably) combine them.

Edit: A few other answers have mentioned de-normalizing--this assumes the relationship between the tables is 1:n (I read your question to mean the relationship was 1:1). If the relationship is indeed 1:n, you absolutely want to keep them as two tables.

Answer 2

Joins themselves are not inherently bad; RDBMS are designed to perform joins very efficiently—even with millions or hundreds of millions of records. Normalize first before you start to de-normalize, especially if you're new to DB design. You may ultimately end up incurring more overhead maintaining a de-normalized database than you would to use the appropriate joins.

As to your specific question, it's very difficult to advise because we don't really know what's in the tables. I'll throw out some scenarios, and if one matches yours, then great, otherwise, please give us more details.

1. If there is, and will always be a one-to-one relationship between user and user_details, then user details likely contains attributes of the same entity and you can consider combining them.

2. If the relationship is 1-to-1 and the user_details contains LOTS of data for each user that you don't routinely need when querying, it may be faster to keep that in a separate table. I've seen this often as an optimization to reduce the cost of table scans.

3. If the relationship is 1-to-many, I'd strongly advice against combining them, you'll soon wish you hadn't (as will those who come after you)

4. If the schema of user_details changes, I've seen this too where there is a core table and an additional attribute table with variable schema. If this is the case, proceed with caution.