I’ve been reading about `composite indexes` and I’m slightly confused about ordering. [This documentation](https://docs.oracle.com/cd/B10501_01/appdev.920/a96590/adg06idx.htm) (little less than half way down) says

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> In general, you should put the column expected to be used most often first in the index.
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However, shortly after it says

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> create a composite index putting the most selective column first; that is, the column with the most values. 
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Oracle also says it [here](https://docs.oracle.com/cd/B10501_01/server.920/a96533/data_acc.htm) in other words

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> If all keys are used in WHERE clauses equally often, then ordering these keys from most selective to least selective in the CREATE INDEX statement best improves query performance.
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However, I have found a [SO answer](https://stackoverflow.com/a/2198069) that says differently. It says 

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> Arrange the columns with the least selective column first and the most selective column last. In the case of a tie lead with the column which is more likely to be used on its own.
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The first documentation I referenced says that you should first go by the most often used whereas the SO answer says that should only be for tie breaking. Then they also differ on the ordering. 

This [documentation](http://docs.oracle.com/cd/B19306_01/server.102/b14211/optimops.htm#PFGRF10105) also talks about `skip scanning` and says

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> Skip scanning is advantageous if there are few distinct values in the leading column of the composite index and many distinct values in the nonleading key of the index.
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Another [article](https://oracle-base.com/articles/9i/index-skip-scanning) says 

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> The prefix column should be the most discriminating and the most widely used in queries
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which I believe most discriminating would mean most distinctive.

All of this research still leads me to the same question; should The most selective column be first or last? Should the first column be the most used and only the most selective on a tie-break? 

These articles seem to be contradicting each other, but they do offer some examples. From what I have gathered, it seems to be more efficient for the `least selective column` to be the **first** in the ordering if you are anticipating `Index Skip Scans`. But I'm not really sure if that is correct.