range can be used when a key column is compared to a constant using any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, or IN () operators: If you are wonsering why join type , when retrieving from single table ,Read this Read more here on EXPLAIN. I've been testing out different ideas for optimizing some of the tables we have in our system at work. In this article I discussed the MySQL EXPLAIN keyword, what its output means, and how you can use its output to construct better queries. Why don't chess engines take into account the time left by each player? ref Though MySQL cannot do a range access method on an infinite number of ranges, it can do it allright on a finite number of ranges. Type =all, indicating full table scan; Key = NULL means the index is not used. Jean-Franois Gagn, [29 Sep 2021 12:37] To learn more, see our tips on writing great answers. Using optimizer tracing, I can see that in the beginning, the right index is not chosen (probably because of statistics), but at step reconsidering_access_paths_for_index_ordering, the right index get picked-up but with access_type ref. MySQL Verification Team, [29 Sep 2021 13:07] It lists the tables in the output in the order that MySQL would read them while processing the statement. What happends with the ownership of land where the land owner no longer exsists? index is used. This is because MySQL can only ever use one index per table (per query). Test environment. The following shows example output from running an EXPLAIN statement. Shameer is a passionate programmer and open-source enthusiast from Kerala, India. When you issue a query, the MySQL Query Optimizer tries to devise an optimal plan for query execution. rev2022.11.14.43032. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. So in a nutshell, the mysql optimizer felt it was better/faster to make the change and use the other index. Why would an Airbnb host ask me to cancel my request to book their Airbnb, instead of declining that request themselves? productvariants table consists of different product variants with productCode as reference keys and their prices. What do you do in order to drag out lectures? Hereinafter, the example will be explained. IN SELECT range type key key_len type ref NULL =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE IN () index index2 respective copyright holders.It is not reviewed in advance However there are still open bugs in the general area. typerowsfilteredextra type. The key_len contains the longest key part that was In the following examples, MySQL can use a ref_or_null join to process ref_table: SELECT * FROM ref_table WHERE key_column=expr OR key_column IS . Today I came across a table that tracks every view on each vehicle in our system. ref_or_null. Light Novel where a hero is summoned and mistakenly killed multiple times, System level improvements for a product in a plastic enclosure without exposed connectors to pass IEC 61000-4-2. NULL is no key was used. Connect and share knowledge within a single location that is structured and easy to search. Ethics: What is the principle which advocates for individual behaviour based upon the consequences of group adoption of that same behaviour? Table C has the following data: 1, John! range - only rows from a given range are being accessed, index is used to select them. What is the mathematical condition for the statement: "gravitationally bound"? Is it bad to finish your talk early at conferences? But even if I wrote a better query, the results would still be the same since there are no indexes. The database's optimizer is supposed to find the quickest way to retrieve it. 1, Jack! select the rows. uses only a leftmost prefix of the key or if the key is not a PRIMARY It may not seem like it, but theres a lot of information packed into those 10 columns! That's very quick, and far quicker than scanning the entire table. Discharges through slit zapped LEDs. Thanks for contributing an answer to Stack Overflow! Asking for help, clarification, or responding to other answers. mysql> EXPLAIN extended SELECT MAX (cr.date) AS max_date, MIN (cr.date) AS min_date, cr.fortnight_title AS period , cr.category_group AS 'group . Take a trivial example, a table with three columns: id (primary key), name (indexed), birthday. More often than not, youll be joining a number of tables together and using complex WHERE clauses. To see the information, follow your EXPLAIN query with SHOW WARNINGS. So, in this case, the index will be ignored. Chain is loose and rubs the upper part of the chain stay. If you try the same query, without the previous re-arrangements, simply after adding the indexes, you wouldnt see much of a reduction. Chain is loose and rubs the upper part of the chain stay. This is actually the only case where MySQL can utilize multiple indexes per query. The EXPLAIN statement provides information about how MySQL executes statements. Example : Beware that MySQL's optimizer often makes errors in this.). The help is greatly appreciated. How can I relate this back to what part of the query? You can use an EXPLAIN statement to get information about how SQL statements are run. In this article youll learn what the output of EXPLAIN can be and how to use it to optimize your schema and queries. Thanks for contributing an answer to Stack Overflow! The meaning of "lest you step in a thousand puddles with fresh socks on". The columns returned by the query are: You can also add the keyword EXTENDED after EXPLAIN in your query and MySQL will show you additional information about the way it executes the query. Using EXPLAIN is as simple as pre-pending it before the SELECT queries. I was playing around with multi-column and single column indexes. Also, the date goes at the end because a range scan can only be done efficiently on the last column in an index. Portions of this website are copyright 2001, 2002 The PHP Group, Page generated in 0.021 sec. EXPLAIN works with SELECT , DELETE , INSERT , REPLACE, and UPDATE statements. Description: Hi, Given two queries where the only difference is the usage of USE INDEX(), EXPLAIN reports a "ref" optimisation and a "range" one for each respective query, whereas the choosen key is the same. Hopefully, this example and discussion will help to explain the differences in the timestamp and datetime data types. index_merge Imagine ref as JOIN between A and C: select A.text, C.text where A.ID = C.ID Here you are: pseudo code illustrating the server side processing: We'll first analyze the original query, then attempt to optimize the query and look into the optimized query's execution plan to see what changed and why. I ran the following query: (Execution time for actual select query was .309 seconds), then I change the date in the where clause from '2011-09-07' to '2011-07-07' and got the following explain results, (Execution time for actual select query was .670 seconds). At some point, doing all those seeks is slower than not using the index at all. Content is licensed under CC BY SA 2.5 and CC BY SA 3.0. 1. It can be either a result of the use of a covering index or an index can be used to retrieve rows in a sorted order. ref - Now imagine another table C with columns (id, text) in which id an index but a non UNIQUE one. operators: If you are wonsering why join type , when retrieving from single table ,Read this. MySQL isnt able to make use of the indexes since it has the WHERE clause in the derived result. Should I use the datetime or timestamp data type in MySQL? Connect and share knowledge within a single location that is structured and easy to search. Say it has a lot of data. You often have to play around with indexes (and rewriting queries) much more than you should have to in MySQL because its optimizer has many deficiencies. That means for each record with orderNumber 10101 in the orderdetails table, MySQL was able to directly find the matching record in all other tables using the indexes and didnt have to resort to scanning the entire table. The 3rd one runs fast and, obviously, examines fewer rows. At this point, the execution time is only .6 seconds for the slow query however we only have about 10% of our vehicles in our database. MySQL EXPLAIN SELECT TYPE. How do I import an SQL file using the command line in MySQL? How can I see the httpd log for outbound connections? A recent, not yet fixed, bug is optimizer prefers using index a to ORDER instead of b,a to EQ and ORDER. How can I change outer part of hair to remove pinkish hue - photoshop CC. In the real world, it can be more useful than the scenarios demonstrated here. It doesn't make a difference to the explanation.]. How did the notion of rigour in Euclids time differ from that in 1920 revolution of Math? How can I change outer part of hair to remove pinkish hue - photoshop CC. That means for executing the query, it will scans 7 110 122 326 2996 = 91,750,822,240 records to find the four matching results. The key column in the output row indicates which The MySQL instance version of this article is 5.7, and the table storage engine is InnoDB I reproduced something similar in 5.6.12. This is what you're seeingexcept in your case, there is another key it can fall back on. Should I use equations in a research statement for faculty positions? any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, or IN() It will plan the query, instrument it and execute it while counting rows and measuring time spent at various points in the execution plan. English Tanakh with as much commentary as possible. EXPLAIN is a handy tool that can help you optimize queries. There can be no duplicate values in the column which is indexed existing in a single column unique index. How does clang generate non-looping code for sum of squares? An SQL developer must decide what type of data that will be stored inside each column when creating a table. MySQL Verification Team, [29 Sep 2021 12:39] This type of join appears for indexed columns compared using. Alternatively, we can use an index hint in the form of "USE INDEX". Question: Among them, Type =const means it is found once by index; Key =primary key is used. What video game is being played in V/H/S/99? type - how MySQL joins the tables used. Regards, Jocelyn How to repeat:How to repeat: Are there computable functions which can't be expressed in Lean? Find centralized, trusted content and collaborate around the technologies you use most. Similary, the four TEXT types are TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT. Is it legal for Blizzard to completely shut down Overwatch 1 in order to replace it with Overwatch 2? index_merge the join uses a list of indexes to produce the result set. Not the answer you're looking for? rev2022.11.14.43032. EXPLAINEXPLAIN id/select_type/table EXPLAIN type/key/ref/rows He has experience in web development using Scala, PHP, Ruby, MySQL, and JavaScript. Why is there "n" at end of plural of meter but not of "kilometer". The following table lists the MySQL string data types that come in pairs. The data type of a column defines what value the column can hold: integer, character, money, date and time, binary, and so on. d2) MULTIPLE in case all CHILD(ren) are above 21. How do I get git to use the cli rather than some GUI application when asking for GPG password? MySQL EXPLAIN 'type' changes from 'range' to 'ref' when the date in the where statement is changed? If you ask MySQL to look for Bob's birthday, it can do that fairly quickly: first, it finds Bob in the name index (this takes a few seeks, log(n) where n is the row count), then one additional seek to read the actual row in the data file and read the birthday from it. In the first rows output you can see the join type used is const, which is the fastest join type for a table with more than one record. how does mysql allocate the key length of a primary key or foreign key? MySQL Verification Team, [30 Sep 2021 14:51] EXPLAIN returns a row of information for each table used in the SELECT statement. In MySQL, numeric data types are categories into two types, either signed or unsigned except . key_len: How many bytes of the key that was used (shows if we are using only parts of the multi-column key). The above query is very slow! Most importantly, the rows column shows MySQL scans all of the records of each table for query. Mobile app infrastructure being decommissioned. Unique Unique is a type of MySQL Index which specifies that all values of the tables columns, when implemented, have to be distinct. TODO: other differences Attached Conditions are Prettier This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. To learn more, see our tips on writing great answers. type to the worst: All rows with matching index values are read from this table for each If there any issues, contact us on - solved dot hows dot tech\r \r#MySQLEXPLAINtypechangesfromrangetorefwhenthedateinthewherestatementischangedSQL #MySQL #EXPLAIN #type #changes #from #range #to #ref #when #the #date #in #the #where #statement #is #changed #- #SQL\r \rGuide : [ MySQL EXPLAIN type changes from range to ref when the date in the where statement is changed - SQL ] His specific areas of interest include cloud computing, and system and database administration. When execution finishes, it will print the plan and measurement instead of the query result. Children of Dune - chapter 5 question - killed/arrested for not kneeling? This means just adding an index isnt always enough; MySQL wont be able to use it unless you write your queries accordingly. The following list describes the join types, ordered from the best By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Let me try to fill the gaps: Index range scan on sbtest1 using idx3 - this part is quite trivial, it says it is going to use a range scan on sbtest1 table and will use the idx3 index. refMySQLNULL MySQLref_or_nullref_tables SELECT * FROM ref_table WHERE key_column=expr OR key_column IS NULL; (6). This is the fastest type of join because the table only has to be read once and the columns value can be treated as a constant when joining other tables. Only rows that are in a given range are retrieved, using an index to Now lets take a look at how we can optimize a poorly performing query by analyzing the output of EXPLAIN. I known the ordinary 'range' type operation.But in this SQL,I have two equal conditions,should be 'ref' (when I use hint,the execution plan ok), mysql sql plan ,what type=range meaning in this sql. So its still faster to find where the Zs start in the name index, then for each one seek the data file to read the row. MySQL's official website explained very simple, only 3 words: . Its purpose is to explain that what the optimizer predicts is the best "plan" and describe that to us. tags: mysql. Question / answer owners are mentioned in the video. Next, consider doing a name like 'Z%'. ref: The reference that is . MySQL Verification Team, [29 Sep 2021 20:31] When ordering by date desc, "Using temporary" slows down query, SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'xxxx' for key 'group_key', MySQL - Optimize Query To Remove Using Temporary and Using filesort, Mysql wrong index after InnoDB compression, How to grow a Dracaena from a broken branch. Thats really horrible, and it will only increase exponentially as the database grows. fulltext - The join is performed using a FULLTEXT index.. ref_or_null - This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values.. index_merge - this join type indicates that the Index Merge optimization is used.. unique_subquery - is just an index lookup function that replaces the subquery completely for better efficiency. index - full index scan is performed. If the key that is used matches Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. you omit the WHERE clause, all records in the table will be updated! Possible values are: system, const, eq_ref, ref, fulltext, ref_or_null, index_merge, unique_subquery, index_subquery, range, index or all. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The status of products and productlines is checked after the UNION, so moving them inside the UNION will reduce the number of records. From MySQL's documentation: When you precede a SELECT statement with the keyword EXPLAIN, MySQL displays information from the optimizer about the query execution plan. It lists the tables in the output in the order that MySQL would read them while processing the statement. Type =ref, because this is considered to be multiple matching rows, in a federated query, is typically ref. The venerable EXPLAIN command has been with us a long time as a built-in MySQL utility statement. Database version (select version(); . How Analytics Can Explain Your Abandoned Checkouts, MySQL Performance Boosting with Indexes and Explain, How to Explain Social Media to a Sixth Grader, DEPENDENT SUBQUERY a subquery which is dependent upon on outer query, UNCACHEABLE SUBQUERY a subquery which is not cacheable (there are certain conditions for a query to be cacheable), system the table has only zero or one row. key changed from vehicleViewTrackingKeyCreatedIndex to vehicleViewTrackingKeyIndex, key_len changed from 50 to 47 (caused by the change in key), rows changed from 23086 to 53676 (caused by the change in key). I searched for someone having the same/similar issue with a date causing this change and was not able to find anything. The key column of. (This is a range scan). Discharges through slit zapped LEDs. While not working, Shameer spends his time coding personal projects, learning, watching screen casts, blogging, etc. If I missed a previous post, please link me :-). Also, the date goes at the end because a range scan can only be done efficiently on the last column in an index. MySQL Verification Team. by Oracle and does not necessarily represent the opinion of type: How rows are found from the table (join type). The video demonstrates the study of programming errors and guides on how to solve the problem.\r\rNote: The information provided in this video is as it is with no modifications.\rThanks to many people who made this project happen. It's getting late and I may have overlooked something in the mysql docs but I can't seem to find why the key (and in turn the type and rows) are changing when the date is changed in the where clause. Timestamp references displayed by the system are UTC. The syntax below shows an SQL query using the EXPLAIN command for MySQL: EXPLAIN SELECT * FROM `user` ORDER BY `id` DESC Reminder: in this example, the query will return information about the execution plan, but will not display the "real" results of the query. As you can see in the result, now the number of approximate rows scanned is significantly reduced from 2,625,810 (219 110 109) to 276 (12 23), which is a huge performance gain. EXPLAIN is one of the most powerful tools at your disposal for understanding and optimizing troublesome MySQL queries, but its a sad fact that many developers rarely make use of it. This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Asking for help, clarification, or responding to other answers. 2022, Oracle Corporation and/or its affiliates, [28 Sep 2021 22:06] The ref column is NULL for this type. used. The EXPLAIN command provides information about how MySQL executes queries. index the entire index tree is scanned to find matching rows. range can be used when a key column is compared to a constant using EXPLAIN EXTENDED can be used to provide . [InnoDB actually has another layer of indirection, I believe, but it'd just confuse the point. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. MySQL Data Types (Version 8.0) Each column in a database table is required to have a name and a data type. That's probably around half the data. 2explain SQL. Stack Overflow for Teams is moving to its own domain! How many concentration saving throws does a spellcaster moving through Spike Growth need to make? If the column . By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Being accessed, index is not used types, either signed or unsigned except with Overwatch 2 `` n at! In pairs with the ownership of land WHERE the land owner no longer exsists of apply... Recaptcha and the Google Privacy policy and cookie policy using only parts of the query, MySQL... Null ; ( 6 ) seeingexcept in your case, the MySQL string data types categories... After the UNION will reduce the number of records account the time left by each player )... The time left by each player allocate the key that was used ( shows if are. Can use an index hint in the form of & quot ; a trivial example, a that... Multiple in case all CHILD ( ren ) are above 21 computable functions which ca n't expressed! When creating a table / logo 2022 Stack Exchange Inc ; user contributions licensed under CC BY-SA on vehicle. When creating a table that tracks every view on each vehicle in our system at work around multi-column! Statements are run to what part of hair to remove pinkish hue photoshop... How do I get git to use it to optimize your schema and queries use it optimize. Explain type/key/ref/rows He has experience in web development using Scala, PHP, Ruby, MySQL, and UPDATE..: SS & # x27 ; YYYY-MM-DD HH: MM: SS & # x27 ; s official explained... The UNION, so moving them inside the UNION, so moving them inside the UNION, so moving inside! Explain works with SELECT, DELETE, INSERT, REPLACE, and JavaScript:! Outbound connections how can I relate this back to what part of hair remove! Expressed in Lean timestamp and datetime data types we have in our system at work `` gravitationally bound?! Stack Exchange Inc ; user contributions licensed under CC BY-SA Z % ' simple! Range are being accessed, index is not used is supposed to find matching.... Union, so moving them inside the UNION, so moving them inside the UNION reduce! Mysql Verification Team, mysql explain type ref vs range 29 Sep 2021 22:06 ] the ref column is compared to a using. From the table will be stored inside each column when creating a.. See our tips on writing great answers so, in this. ) REPLACE with. A spellcaster moving through Spike Growth need to make the change and use other. This. ) n't chess engines take into account the time left by each player built-in MySQL statement! Corporation and/or its affiliates, [ 29 Sep 2021 22:06 ] the ref column compared. Through Spike Growth need to make `` n '' at end of plural of meter but not of `` ''... For outbound connections 1920 revolution of Math this website are copyright 2001, the..., a table book their Airbnb, instead of the records of table. Across a table that tracks every view on each vehicle in our system at work end! Ss & # x27 ; format you omit the WHERE clause, all records in the real world it! Watching screen casts, blogging, etc condition for the statement: `` bound!, see our tips on writing great answers this article youll learn what the output of EXPLAIN can used. Replace, and LONGTEXT felt it was better/faster to make the change and was not able to it. It has the WHERE clause, all records in the timestamp and datetime types... Is used index is used is loose and rubs the upper part of the multi-column key ) Lean! 12:37 ] to learn more, see our tips on writing great answers is compared to a using... Single table mysql explain type ref vs range Read this. ) application when asking for GPG password exponentially the! Inside the UNION will reduce the number of records decide what type of appears. How SQL statements are run non UNIQUE one key or foreign key a given range are being,! Its affiliates, [ 29 Sep 2021 12:37 ] to learn more, see our tips writing. Copyright 2001, 2002 the PHP group, Page generated in 0.021 sec means the index at.. A federated query, it can fall back on hair to remove pinkish hue - CC! Can only be done efficiently on the last column in a thousand with... Quicker than scanning the entire mysql explain type ref vs range outer part of the key length of a primary key or key. Blogging, etc part of hair to remove pinkish hue - photoshop CC Version 8.0 each! Chain stay really horrible, and JavaScript httpd log for outbound connections their. Is indexed existing in a nutshell mysql explain type ref vs range the date in the real world, it will print the and. Is structured and easy to search each player site is protected by reCAPTCHA and the Google Privacy policy Terms. N'T make a difference to the explanation. ] book their Airbnb, instead of declining that themselves! Indirection, I believe, but it 'd just confuse the point lest. Clicking post your answer, you agree to our Terms of Service, Privacy and... Our Terms of Service, Privacy policy and cookie policy ; use index & quot ; use &. Beware that MySQL 's optimizer is supposed to find matching rows, in.! Importantly, the MySQL string data types ( Version 8.0 ) each column in a research statement faculty. Moving through Spike Growth need to make use of the chain stay * ref_table! From ref_table WHERE key_column=expr or key_column is NULL ; ( 6 ),,... Are using only parts of the query, the date goes at the end because range! Using the index is not used found from the table ( join type, when retrieving single! The WHERE statement is changed I came across a table that tracks every view on each in. I change outer part of hair to remove pinkish hue - photoshop CC from that in 1920 revolution of?... ) multiple in case all CHILD ( ren ) are above 21 for password. Mediumtext, and far quicker than scanning the entire table column in an index isnt always ;! From that in 1920 revolution of Math only increase exponentially as the database 's optimizer is to... The last column in a nutshell, the index is used EXPLAIN command has been us. The notion of rigour in Euclids time differ from that in 1920 revolution of Math the! When a key column is compared to a constant using EXPLAIN is a handy tool that can you. Of declining that request themselves the output of EXPLAIN can be more useful the., DELETE, INSERT, REPLACE, and it will scans 7 110 326! To use it to optimize your schema and queries I use the datetime timestamp... And/Or its affiliates, [ 29 Sep 2021 12:39 ] this type executes.... The only case WHERE MySQL can utilize multiple indexes per query fewer rows the multi-column key ),.... Text, MEDIUMTEXT, and UPDATE statements table is required to have name! ) are above 21 of tables together and using complex WHERE clauses in. It will print the plan and measurement instead of the records of each table used in the that... Not necessarily represent the opinion of type: how to repeat: are computable! And far quicker than scanning the entire index tree is scanned to find the matching. Sum of squares and their prices a primary key or foreign key how rows are found from the (! Is licensed under CC BY-SA responding to other answers order to drag out lectures used ( shows we! Type, when retrieving from single table, Read this. ) with coworkers Reach..., PHP, Ruby, MySQL, and LONGTEXT scan can only ever use one index per table ( type. And far quicker than scanning the entire index tree is scanned to find matching rows type/key/ref/rows He experience... It legal for Blizzard to completely shut down Overwatch 1 in order to drag out lectures ) in which an... I wrote a better query, is typically ref knowledge within a single that. Retrieves and displays datetime values in & # x27 ; s official website very. I use the other index one runs fast and, obviously, examines fewer rows, this example discussion. Is another key it can be and how to repeat: are there computable functions ca. `` n '' at end of plural of meter but not of `` kilometer.... Together and using complex WHERE clauses with us a long time as a MySQL... Command line in MySQL but a non UNIQUE one of products and productlines is checked after the,... And using complex WHERE clauses one index per table ( per query ) happends the!. ) Sep 2021 22:06 ] the ref column is NULL ; ( 6 ) time. =Primary key is used to provide is not used InnoDB actually has another of... Please link me: - ) MEDIUMTEXT, and LONGTEXT it bad to finish your talk early at?. To optimize your schema and queries SQL statements are run often than not, youll be a... Retrieves and displays datetime values in the WHERE clause, all records in the real,! Another layer of indirection, I believe, but it 'd just confuse the point only parts of multi-column... Explain 'type ' changes from 'range ' to 'ref ' when the date in table! Your queries accordingly differences in the real world, it will scans 110!
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