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    <p>If you only have 1 row in the table, the cardinality for the index should be 1, of course. It's just counting the number of unique values. </p> <p>If you think of an index as a lookup-table based on buckets (like a hash), then the cardinality is the number of buckets.</p> <p>Here's how it works: When you build an index over a set of columns <code>(a,b,c,d)</code>, then the database goes over all the rows in the table, looking at the ordered quadruplets of those 4 columns, for each row. Let's say your table looks like this:</p> <pre><code>a b c d e -- -- -- -- -- 1 1 1 1 200 1 1 1 1 300 1 2 1 1 200 1 3 1 1 200 </code></pre> <p>So what the database looks at is just the 4 columns (a,b,c,d):</p> <pre><code>a b c d -- -- -- -- 1 1 1 1 1 2 1 1 1 3 1 1 </code></pre> <p>See that there are only 3 unique rows left? Those will become our buckets, but we'll get back to that. In reality, there's also a record id, or row identifier for each row in the table. So our original table looks like this:</p> <pre><code>(row id) a b c d e -------- -- -- -- -- -- 00000001 1 1 1 1 200 00000002 1 1 1 1 300 00000003 1 2 1 1 200 00000004 1 3 1 1 200 </code></pre> <p>So when we look at only the 4 columns of (a,b,c,d), we're really looking also at the row id:</p> <pre><code>(row id) a b c d -------- -- -- -- -- 00000001 1 1 1 1 00000002 1 1 1 1 00000003 1 2 1 1 00000004 1 3 1 1 </code></pre> <p>But we want to do lookup by (a,b,c,d) and not by row id, so we produce something like this:</p> <pre><code>(a,b,c,d) (row id) --------- -------- 1,1,1,1 00000001 1,1,1,1 00000002 1,2,1,1 00000003 1,3,1,1 00000004 </code></pre> <p>And finally, we group all the row ids of rows that have identicle (a,b,c,d) values together:</p> <pre><code>(a,b,c,d) (row id) --------- --------------------- 1,1,1,1 00000001 and 00000002 1,2,1,1 00000003 1,3,1,1 00000004 </code></pre> <p>See that? The values of (a,b,c,d), which are (1,1,1,1) (1,2,1,1) and (1,3,1,1) have become keys for our lookup table into the rows of the original table.</p> <p>Actually, none of this really happens, but it should give you a good idea on how a "naive" (i.e. straight-forward) implementation of an index might be done.</p> <p>But the bottom line is this: cardinality just measures how many unique rows there are in an index. And in our example that was the number of keys in our lookup table, which was 3.</p> <p>Hope that helps!</p>
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    1. POWhy does the cardinality of an index in MySQL remain unchanged when I add a new index?
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    1. USShalom Craimer
    1. USShalom Craimer
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    1. COThanks for the index info. Very well explained. The cardinality of my index should be more than 1 given that there are 30000 rows and almost every member has a different name?
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    2. COThanks for the explanation about indexes, it was very nice, but your explanation didn't answered the above asked question .
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      1. USDeepak Yadav
    3. COYou're right, I didn't explicitly say the final conclusion: I just showed that 4 rows fall into 3 buckets. I'm sure you could invent another row that could be added that would fall into one of the existing 3 buckets of the index. That would leave the number of buckets unchanged, which would also mean the cardinality of the index was unchanged. Sorry about that.
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      1. USShalom Craimer
 

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