What does it mean to "reduce the selectivity"?

Hello guys

I try to understand the following

As the columns are compressed from left to right, the columns must be ordered in decreasing selectivity to get the best compression.

As far as I know, selectivity = number of distinct values / num lines

So, if selectivity diminishes, the number distinct values decreases, right?

Question 2: Correct me if I'm wrong, also a means of selectivity, if the sql query returns multiple rows, it is said that low selectivity. However, if the sql query returns multiple rows, it is said that strong selectivity.

Thanks in advance.

Talk you about the compression of the table or index of compression?  Or something else entirely?

In the context of compression, if I read the line you quote, my interpretation of "lower selectivity" is "the column with the distinct values least is on the left, the column with the most distinct values concerning the right of it... and so on.

Hemant K Collette

Tags: Database

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