Which Order Should Columns Go in an Index?: Finding All the Red Sweets Part 4

When you create an index on multiple columns there's an important question you need to answer:

In which order should you list the columns?

This video looks at some of the factors you should consider to help answer this question.

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The Magic of SQL with Chris Saxon

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[cyberplayer]

this video answers my two questions.
1. how multiple columns index works? scan through the leading index column first and then to the next index column.
2. why is there must be a primary key index? because the index table size is smaller than the content table itself, that said the query can performs better.

Thanks alot!

[satyendrar]

So, A column with low cardinal values should be the leading column in the index?

[ParleysWoodedJourney]

How does that compare to two separate indexes; one by color and one by weight?

[jb]

Are you going to try to cover index fast full scans (vs. index full scans)? I'm not sure how you'd go about it...