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, found 1 matches left After the join standard row, no more rows are checked in the table for the previous row combination. Range checked for each record (Index map: #): MySQL did not find a good index to use, but found that if the column values from the previous table were known, some indexes might be available. The Using Filesort:mysql requires an additional pass to find out how rows are retrieved in sort
are checked in the table for the previous row combination. Range checked for each record (Index map: #): MySQL did not find a good index to use, but found that if the column values from the previous table were known, some indexes might be available. The Using Filesort:mysql requires an additional pass to find out how rows are retrieved in sorted order, indicating that the
Label:1. First check whether the profiling function is turned on like ' %pro% 'OrSELECT @ @profiling2. Open ProfilingSET Profiling=13. Execute SQL statementsFor example:SELECTTable_schema as 'Db Name', ROUND(SUM(Data_length+Index_length)/ 1024x768 / 1024x768,3) as 'Db Size (MB)', ROUND(SUM(Data_free)/ 1024x768 / 1024x768,3) as 'Free Space (MB)' fromInformation_schema.tablesGROUP byTable_schema;4. View ResultsSHOW profiles; SHOW Profile All f
can have no keys and rows are not necessarily unique, in which case the table is not a collection, but a multiset (multiset) or package (bag). But even if the table being queried has a primary key and meets the conditions of the collection, a select query against the table may still return a result that contains duplicates. The result set is often used when describing the output of a select
1: Find the largest maximumSelect Max (Salary) from the employee where salary2. SortingSelect Salary from employee where Salary not in (select MAX (Salary) from employee) Order BY Salary desc LIMIT 1; Select (select DISTINCT Salary from Employee order by Salary Desc limit 1 offset 1) as secondheighestsalary; Find Nth Number:CREATE FUNCTION getnthhighestsalary (N INT) RETURNS intbegindeclare M int;set n=n
example:In this table, CITY_ID is the primary key, the City field is a varchar type, the length is 50, the default is null, and the explain select city from Sakila.city is executed as follows:Can be found here to go to overwrite the index, by the way, the overwrite index is the SQL query content through the SQL index to be able to
Tags: sql thinkphp phpA Query methodthinkphp provides three basic query methods: String conditional query, indexed array condition query, and object barQuery. In most cases, it is recommended to use indexed arrays and object methods as q
ERROR OGG-00446 Oracle GoldenGate Capture for Oracle, Extjms2.prm:Could don't find archived log for sequence 1 thread 2 Under Default destinations SQL The goldengate extraction process is stopped because the archive log for thread 2 is 1 has been deleted:Workaround:1, restore the deleted archive log, restart the extraction process;Rman>restore Archivelog from Logseq 1 until Logseq 2;Ggsci>start EXTJMS22, th
Original: Remember the importance of a T-SQL query to optimize indexesOverviewWhen tuning the performance of a project component, it is really important to find the SQL design, so write a blog record to summarize.Environment IntroductionThis project component is a window service that internally uses the round-robin opp
What is an index? Index is an important tool to improve query performance. index is to add a small amount of data required by the query statement to the index page. In this way, you only need to access the pages of a few indexes when accessing data. However, indexes are not omnipotent in improving query performance, nor are they as good as creating more indexes.
Label:Use databaseDECLARE @ProcName varchar (50)Create Table #tmpName (Content varchar (2000))Create Table #tmp (procname varchar, Content1 varchar (8000))--Define a cursorDECLARE Searchproc CURSOR for--Query the name of the stored procedure in the database, as far as possible to remove the system proc, can be based on the Crdate time field to find non-system procSelect name from sysobjects where type= ' P
optimization. Based on a cost evaluation model, in the process of generating the query plan, the cost of each access path (the access path mainly includes the above three "relationship nodes") is calculated, and then the least cost is chosen as the sub-path, so that all tables are connected to a complete path. The mainstream database uses rules-based and cost-based technologies. 4) Parallel qu
fine. Then write the SQL statement to the C # program, the result of a query statement when the error jumped out, baffled by the solution. So find the Access Help file and find the following help: //////////////////////////////////////////////////////////// Compares a string expression with a pattern in an
General Fuzzy query statements are as follows: Copy Code code as follows: SELECT field from table WHERE a field like condition With regard to conditions, SQL provides four matching modes: 1,%: Represents any 0 or more characters. Can match any type and length of characters, in some cases in Chinese, please use two percent sign (%). For example, SELECT * from [user] WHERE u_name l
(); /* * Native SQL statement query, will find all fields of t_student table, stored in a object[] array * If you want to convert to a solid object, you only need to call Addentity (Student.class ) can, at this time, will first match * Student object inside the property whether all query out, if not, then error (If th
Label:Recently often see the word "parameter sniffing", read a few articles, so I try to do a test to deepen the impression! Go to the official website to download the database: AdventureWorks2012 Direct Test it! Find a few familiar tables to associate with, Use ProductID as a condition to find two IDs returns a value that has a large difference in the number of rows. productid=870 (4688 lines) productid=89
A few days to a company in Pudong to interview, feel that there is a database query questions did not answer well, self-summary, the next attention.Title: The size of tables in the database, and arranged out. Write the corresponding SQL statement.The code is as follows:CREATE PROCEDURE [dbo].[Gettablespace]@dbNamesysname= "',--database name, default current database@tableNamesysname= "',--table name, defaul
section, the reasonable use of column storage indexes for large data can improve query performance, but by looking for a large amount of data found in SQL Server There are a number of limitations and problems with the use of column storage indexes in 2012, and the problem with SQL Server 2014+ is greatly improved, so it is not recommended that you use column sto
rows with different indexes can run the following SQL statements:StatementSelect count (*) from recordWhere date> '123' and date Statement BSelect count (*) from recordWhere date> '2013' and place in ('bj ', 'sh ')Statement CSelect date, sum (amount) from recordGroup by date1. Create a non-clustered index on DateA: (25 seconds)B: (27 seconds)C: (55 seconds)Analysis:There are a large number of duplicate values on date. In a non-clustered index, data i
Label:Original: SQL Server Tuning Series advanced (how the query optimizer runs)ObjectiveIn the previous articles, we introduced a series of basic descriptions of operators and the optimization methods and techniques for each operator. This includes a series of our common operators, including how to view execution plans, how several datasets are commonly used, how to join operators, and parallel operators.
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