Depending on thedetails of your tables, columns, indexes, and the conditions in your WHEREclause, the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving seve...
unionM,N: The row refers to the union of the rows with id values of M and N. derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. subqueryN: The row refers to the...
<derivedN>:ID值n的行的派生表结果,派生表可能是从FROM子句中的一个子查询生成的(The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause.)。 <subqueryN>:ID值n的行的物化子查询结果。 4....
Forstorage engines that maintain an exact row count per table (such as MyISAM, butnot InnoDB), this Extra value can occur for COUNT(*) queries for which the WHERE clause is missing or always true and there isno GROUP BY clause. (This is an instance of an implicitlygrouped query where ...
If you want to query execution plans with higher readability, we recommend that you upgrade your Hologres instance to V1.3.4x or later. EXPLAIN Syntax You can execute the following EXPLAIN statement to query the execution plan that is estimated by the QO: EXPLAIN <sql>; Example In this ...
SQL- Where Clause: Answer and Explanation: The purpose of ''Where'' clause in a query is to specify, to limit and to filter a condition when getting data from a table. Tables can be either be... Learn more about this topic:
Some of these factors include: » Data skew » Multiple single column predicates on a single table » Function wrapped columns in the WHERE clause predicates » Complex expressions In the previous example there is a data skew in the EMPLOYEES table. There is not an even number of ...
Now, let us check the followingExplain Query Planwith SELECT statement − SQLite>EXPLAINQUERYPLANSELECT*FROMCOMPANYWHERESalary>=20000;orderfromdetail--- --- ---00TABLECOMPANY Print Page Previous Next
Cause: A GROUP BY column in the query is not present in the GROUP BY clause of the materialized view. This usually requires a join back to a third table. Action: No action required. QSM-01059 HAVING clause in the query block is malformed Cause: An illegal HAVING clause has been specif...
Description: For queries with HAVING clause EXPLAIN may not show real plan used (suggesting other one). This makes troubleshooting more complex. Here is a simplified example (based on real life case where query execution time was very different, while EXPLAIN had not let to see why), on ...