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result set type returned by the subquery is a simple value.
b) Single-row sub-query.
The result set type returned by the subquery is 0 or one unit group. Similar to the scalar subquery, but may return 0 tuples.
c) Multiline single-row subquery.
The result set type returned by a subquery is a multi-tuple but has only one simple column.
d) Table sub-
31514.27 optimizing multiple subqueries with LEFT JOIN (i) 31714.28 optimizing multiple subqueries with LEFT JOIN (ii) 32014.29 optimizing multiple subqueries with LEFT JOIN (iii) 32214.30 removing the filter 324 caused by the exists14.31 Overlap Time Count 32514.32 overriding optimizations with analytic functions 32814.33 parts Suppliers of equal set 33414.34 Hangar of equal sets with pilot 33514.35 overriding the maximum filter condition with an analytic function 33814.36 finding data at a sp
and xh_bz, while in the second SQL 0.5% records are DY_DJ and xh_bz comparisons, so that the second SQL CPU utilization is significantly lower than the first one.3. Influence of query Table orderThe order of the list in the table following the from will have a performance impact on SQL, and with no indexes and no stat
SQL Server performance optimization skills, SQL Server Performance Optimization
1. Select the most efficient table name sequence (only valid in the rule-based Optimizer)
The SQL
there are usually several ways to tune a database:
1) Manual tuning.
The
main reliance on people, inefficient, requiring the operator to fully understand the principle of common sense depends on, but also the application, database management system, operating system and hardware have a broad and deep understanding.
2) Case-based tuning .
This
paper summarizes the recommended configuration values of database parameters and the design of
SQL Server address search performance optimization and SQL server performance optimization
This is an example of a long time ago. Now, I have no intention of discovering the materials, so I will take it out and try again.
1. Requi
MySQL's subquery optimization has been not very friendly, has been criticized by the industry more than I have encountered in the SQL optimization of one of the most problems, you can click here, here to get some information, MySQL processing subqueries, the subquery will be rewritten, usually, we hope that from the inside out, That is, the result of the subquery
table and prefix the alias to each column. This reduces the time to parse and reduces the syntax errors caused by column ambiguity.(15) Replace in with exists with not exists instead of in:In many base-table-based queries, it is often necessary to join another table in order to satisfy one condition. In this case, using EXISTS (or not EXISTS) will usually improve the efficiency of the query. In a subquery, the NOT IN clause performs an internal sort
certain "big" sqlFor a disruptive, high concurrency SQL will always be larger than the low frequency, because the high concurrency of SQL will not even give us any respite when it comes to the problem. For some SQL, which consumes a lot of IO and is slow to respond to, because of the low frequency, even if encountered, the most is to allow the entire system to s
, the use of >= instead of, for example:SELECT * FROM table where hh>10;//inefficient select * from table where hh>=10.0000001//relatively efficientII. Optimization of SQL statementsDescribes the optimization techniques for several SQL statements:(1) The Order of links in th
Artificial Intelligence automatic SQL optimization tools-SQLTuning for SQL Server, SQL-sqltuning
In response to this situation, the artificial intelligence automatic SQL optimization to
Tags: SQL optimizationThe most common way to improve the efficiency of SQL statements is to build indexes and avoid full table scans as much as possible. Let's tidy up some common SQL optimization techniques to avoid full table scanning. A simple
example:
SELECT/*+full (BSEMPMS) Cahe (BSEMPMS) * * Emp_nam from Bsempms;
/*+nocache (TABLE) * *
When a full table scan is performed, the cache hint can place the retrieved block of the table in the buffer cache with the most recent LRU of the least list
For example:
SELECT/*+full (BSEMPMS) Nocahe (BSEMPMS) * * Emp_nam from Bsempms;
/*+append*/.
You can increase the speed by inserting it directly into the end of the table.
Insert/*+append*/into test1 select
a copy of the original table, but there is only one member table and one distributed partitioned view on each server. The location of the data is transparent to the application.11. Rebuild the index DBCC REINDEX, DBCC INDEXDEFRAG, shrink data and log DBCC SHRINKDB,DBCC shrinkfile. Sets the auto-shrink log. For large databases do not set the database autogrow, it will degrade the performance of the server.
MS SQL Server Query optimization methodThere are many reasons for the slow speed of queries, which are common1, no indexes, or no indexes (this is the most common problem with query slowness, is the flaw in programming)2, I/o throughput is small, creating a bottleneck effect
= rcvlbes.customer_id and rcvblls.balance>0 ORDER by Cust.name Into TEMP cust_with_balance Then query in the temporary table in the following way: SELECT * from Cust_with_balance WHERE postcode> "98000" The rows in the staging table are less than the rows in the primary table, and the physical order is the required order, reducing disk I/O, so the query effort can be significantly reduced. Note: Changes to
field c is not used in the index and needs to be retrieved from the table. Under such circumstances, the IO volume of the two will be significantly different.
4. order by must be sorted.
We know that the index data is actually ordered. If the data we need is consistent with the order of an index, and our query is executed through this index, generally, the database will omit the sorting operation and directly return the data because the database kn
Tags: Wildcard time example log action single quote use Cape includesReprint: http://new-restart.iteye.com/blog/1416765 SQL like wildcard character fuzzy query techniques and special charactersSQL like wildcard character fuzzy query techniques and special characters How to d
table where substring (name,1,8) = ' Uncletoo 'or select ID from table where DATEDIFF (Day,datefield, ' 2014-07-17 ') >= 0The fields are function-treated in these two statements, so that the query parser discards the use of the index. The correct wording is this:Select ID from table where name is like ' uncletoo% 'or select ID from table where Datefield In other words, do not perform functions, arithmetic operations, or other expression operations on
resulting set of results will be sorted after the table is connected, the duplicate records are deleted and the results returned. Most of the actual applications do not produce duplicate records, the most common being the process table and the history table UNION. Such as:SELECT * FROM Gc_dfysUnionSELECT * FROM Ls_jg_dfysThis SQL takes out the results of two tables at run time, then sorts the duplicate records with the sort space, and finally returns
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