| 2 | DEPENDENT SUBQUERY | EVENTS | NULL | ref | EVENTS_USER_IDX | EVENTS_USER_IDX | 5 | | 97 | 10.00 | Using where |Īlmost identical: both select rows from the USERS table, and then use a nested-loops join (“dependent subquery&rquo ) to retrieve rows from the EVENTS table. | 2 | DEPENDENT SUBQUERY | EVENTS | NULL | index_subquery | EVENTS_USER_IDX | EVENTS_USER_IDX | 5 | func | 195 | 10.00 | Using where | | 1 | PRIMARY | USERS | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 5 | 100.00 | Using where Using index | | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | The first plan is from the NOT IN query, the second is from the NOT EXISTS. This version runs in 0.01 seconds, which is more what I expected. Let's replace that NOT IN with a NOT EXISTS and correlated subuery: select ID I'm running on my laptop, which has a Core i7 processor, 12 GB of RAM, and an SSD.Īnd I consistently get runtimes of around 2 minutes, which is … wow. Using a NOT IN, and ensuring that nulls don't appear in the inner results, the query looks like this: select IDįor my test dataset, the USERS table has 100,000 rows and the EVENTS table has 10,000,000, of which approximately 75% have a null USER_ID. OK, now let's use these tables: From a small set of users, we want to find the ones that haven't had a particular event. create table USERSĬreate index EVENTS_USER_IDX on EVENTS(USER_ID) However, when the user is not null, the secondary index has high cardinality. Since we track both anonymous and registered users, EVENTS.USER_ID is nullable. This post is about one case where it's dramatically slower, and nulls are to blame.Ĭonsider the following two tables, which might be used to track clickstream data. But if you guard against that, they should be equivalent - indeed, some sources will tell you that NOT IN is faster and therefore preferred. There is one key difference between the two constructs: if the subquery returns a NULL in its results then the NOT IN condition will fail, because null is neither equal-to nor not-equal-to any other value. And modern database systems will optimize the two queries into similar execution plans, handling the correlation between outer and inner queries (I say “modern” because when I was working with Oracle 7.3 in the mid-90s I learned the hard way that it did not). The latter is arguably simpler to write and makes the intent of the query more obvious. How can I get a users that exceed $ 1000 in the last 30 days? for each user last 30 days.When you want to perform a difference operation between two tables, you have a choice: NOT EXISTS with a correlated subquery, or NOT IN.How to find median given frequency of numbers?.How to upload file into folder after saving form.Replace Spaces in Column Name with Underscore MySQL.Trying to get total appointments per state.Will lots of small sql statements slow my site down significantly.mySQL use row in table to find multiple rows in second table quickly.How to Access User Email and Password from MySql PhpMyadmin Local host.Using delimiters with multiple functions in MySql.ORDER BY FIELD (column_name, (SELECT )).How to make user friendly column headings.Mysql update query limit not working if i join two tables.Mysql query which do not display rows with not equal value.SQL statement will not work in PHP but works in MySQL.
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