The SQL query using Count in HAVING clause. Looking at how people are using COUNT(*) and COUNT(col) it looks like most of them think they are synonyms and just using what they happen to like, while there is substantial difference in performance and even query result.

Costs are visible only in TREE format (since 8.0.18) and JSON (used in MySQL Workbench for visual explain plans). EXPLAIN SELECT COUNT(*) FROM tbl_name could incorrectly report an Extra value of "Select tables optimized away" due to a modification introduced in MySQL 5.7.2 that caused InnoDB to count rows by traversing the clustered index instead of a smaller secondary index.

In my previous blog post, we learned that indexes or other means of finding data might not be the most expensive part of query execution.For example, MySQL GROUP BY could potentially be responsible for 90% or more of the query execution time. For a simple query, it would look like the following: mysql count explain. The venerable EXPLAIN command has been with us a long time as a built-in MySQL utility statement. In this blog post, I’ll look into four ways MySQL executes GROUP BY. A MySQL EXPLAIN number of rows discrepancy.

In today’s tip, we’ll use the native COUNT() function to retrieve the number of rows within one table or view within a MySQL … This is also an issue for me on Amazon RDS MySQL 5.7.16 after upgrading from MySQL 5.6: Large table (21 million rows) with 21 indexes. > …just a simple count query, it does not give us the time for that step or how many rows were read or which index was used… MySQL explains how it would process the statement (emphasis mine). As the manual says, .
28 Oct 2019. There are several ways to get a row count in MySQL. The higher count means better selectivity and you have to arrange fields in the index from the best selectivity to worst. In MySQL 8.0.18 there is a new feature called Explain Analyze when for many years we mostly had only the traditional Explain. Some database management products provide database statistics like table sizes, but it can also be done using straight SQL.

This is also an issue for me on Amazon RDS MySQL 5.7.16 after upgrading from MySQL 5.6: Large table (21 million rows) with 21 indexes.
share | improve this question | follow | asked Aug 30 '17 at 10:50. In the following SQL query, we will return the rows containing employee names, the number of times salary paid and its sum only for those which salary paid is greater than 3000. The EXPLAIN keyword is used throughout various SQL databases and provides information about how your SQL database executes a query. Moreover, the HAVING clause is used with COUNT function for restricting only those employee who got paid two or more times. Going back to our simple query: it is a SIMPLE type of select with a const type of join.


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