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based on the following criteria: 1, the column is used for frequent searches, 2, the column is used to sort the data, 3, there are fewer duplicate values in the column.The following columns are not suitable for indexing: 1, the number of duplicate values in the column, 2, the low data in the index (indexing for small tables is not necessary, because retrieving the index may take longer than retrieving the index); 3. FrequentInsert the column of the operation (because each time new data is added

overhead is not reduced, and it is still the same as returning all the result sets.2. In an indexed (nonclustered index, to overwrite the return column, this will cause a RID lookup, or a key-value lookup, which will be described in a later article. ）Create an indexCreate nonclustered index index_headers_buydate on headers (buydate)Include (ID, Amount,discount)GoSelect ID, amount,discount,buydate from HeadersSelect ID, amount,discount,buydate from Headers where buydate= ' 2008-09-15 'Result: In

Tags: Operation list remove exp Introduction drop table LTE ValThe partition in MySQL has range, list, hash, key four kinds of partitioning methodsWe introduce the Range method here, and others can digest Partitioning an existing table (range mode) ALTER table name PARTITION by range (field name) (PARTITION partition name values less THAN (field value)); Alter TABLE by range (ID) ( values less than (+), values less than), VALUES Less than (MAXVALUE) ); View partitio

#commit and rollback are used to ensure that the database has enough space left;#commi, rollback can only be used for DML operations, that is, insert, UPDATE, delet;#rollback操作撤销上一个commit, rollback after the transaction.CREATE TABLE Test(prod_id varchar (TEN) is not NULL,Prod_desc varchar (+) NULL,Cost decimal (6,2) null);#禁止自动提交Set autocommit=0;#设置事务特性, must be set before all transactions begin#set transaction Read only; #设置事务只读Set transaction read write; #设置事务可读, write#开始一次事务Start transaction;

mainly used to centralize data from branch office to head office, or distribute it from head office to branch office, if the company needs to synchronize the dataVery few, the other inter-company business system is not the same company developed synchronization function mainly through the database synchronization software implementation.3.oracle table rollback to an action statement at a specified timeALTER TABLE table name enable row movement;Flashback table name to timestamp to_timestamp ('

Dirty Reads (Dirty read) dirty reads mean that one transaction reads uncommitted data from another transaction, and this data is likely to be rolled backNon-repeatable read (unrepeatable read)Non-repeatable reading means that in database access, two identical queries within a transaction range return different data. This is caused by the commit of the other transaction modifications in the system during the query.For example: Transaction B executes two times for a query, and when the first exe

MySQL thing implementation syntax1) Begin SQL statement rollback or commitSTART TRANSACTION SQL statement rollback or commit2) Modify the #本文未涉及 that the autocommit auto-commit is turned onPersonal understanding of this logicStart thingsSubmit each SQL, and record the success or not of the submissionMake judgments on all SQL submission resultsIs successful, commit# commits the transaction, and all operations are performedOtherwise (any one execution fails), then rollback. To perform a transactio

Tags: serialization session lock table serial Repeat commit table BSP CTI1. ViewSELECT @ @tx_isolation2. Settings2.1 All Levels　　1 Read UNCOMMITTED: Reads data that has not yet been committed: no problem is solved 2) Read Committed: Reads the data that has been submitted: can resolve dirty read---- Oracle default 3) repeatable READ: Reread read: Can solve dirty read and Non-repeatable READ---mysql default 4) Serializable: Serialization: Can resolve dirty read non-repeatable read and virtual rea

Tags: io file data SP time r BS Database sqlMethod 1,Set the file growth to a fixed size. such as FileGrowth = 100MB;--------------------------------------------------------------------------------------------------------------- -------------------------------------------Data file growth notes,For example, table T is built on files File1 and file2. File1 file2 each have 500MB of free space. It is very possible to insert 100MB data into a table file1,file1 each insert50MB. But if you only insert

commit is implied after each statement.B: an explicit transactionBegin explicitly with BEGIN TRANSACTION, ending explicitly with commit or rollback.C: Implicit transactionsWhen the previous transaction completes, the new transaction is started implicitly, but each transaction still ends explicitly with commit or rollback.(4): Characteristics of the transaction (acid characteristics)A: atomicity (atomicity)A transaction is a logical unit of work for a database, and all operations included in the

, close the connection Example: (1). Open a database connection The. Connect () function is used to create a connection, write the host, username, password, existing database, port number (2). Create a table (3), inserting data . Cursor () is the method used to get Python to execute the MySQL command, that is, to obtain an operation cursor; The Execute () function is the command used to execute the SQL statement, the. Commit () function is used to commit the transaction,

Tags: nosql other log and key value MongoDB couchdb POS generation==1 nosql== NoSQL, which refers to non-relational databases. NoSQL databases are created to address the challenges of multiple data types in large-scale data sets, especially big data application challenges. Four categories of NoSQL databases 1.1 键值(Key-Value)存储数据库 这一类数据库主要会使用到一个哈希表，这个表中有一个特定的键和一个指针指向特定的数据. 1.2 列存储数据库。 这部分数据库通常是用来应对分布式存储的海量数据。键仍然存在，但是它们的特点是指向了多个列。这些列是由列家族来安排的。 1.3 文档型数据库 该类型的数据模型是版本化的文档，半

Tags: default string One summary database connection itself encounters eating ampProfileThere are always problems when Oracle and EF work together, so here's a summary of the solution.Schema schemaPeople who have used Oracle should know that Oracle's user name is typically its schema name, and if you are building an EDMX file in database First mode, then congratulations, once you change the username you will get an error!So try to avoid using EDMX to use, recommended model first mode, is to chan

Con.setautocommit (FALSE); Set TRANSACTION manual commit/*** First transfer ***/pstmt = Con.preparestatement (sql_zs1);p stmt.executeupdate ();p stmt = Con.preparestatement ( SQL_LS1);p stmt.executeupdate ();//Roll back to this position? SP = Con.setsavepoint (); /*** Second Transfer ***/pstmt = Con.preparestatement (sql_zs2);p stmt.executeupdate ();p stmt = Con.preparestatement (SQL_LS2); Pstmt.executeupdate ();} catch (Exception e) {try {//rollback (rollback to specified code snippet) con.ro

precision of seconds. Indicates that the maximum is date (' y-m-d h:i:s ', 4294967295) up to 2106-02-07 14:28:15, can be null, but the business logic is designed to use a non-null value whenever possible. It has the advantage of numerical storage, space-saving. But the readability is poor. In addition, if the business often needs to arrange or query by time, try to use the int type.4, bigintThis type of storage length is 8 bytes, from -2^63 (-9223372036854775808) to 2^63-1 (9223372036854775807)

MySQL build database or build a table will encounter the problem of choice coding, we used to be a habitual choice utf8, but in MySQL after 5.5.3 version with UTF8MB4 encoding, UTF8MB4 can save 4 bytes Unicode,mb4 is most bytes 4 of the meaning. Including emoji expressions and a lot of infrequently used Chinese characters can be stored, but also can store any new Unicode characters, extensibility good ah.The maximum character length of the UTF8 encoding is 3 bytes, and if it encounters a 4-byte

Tags: mysq row storage data problems post data table appears MySQL popularWhen you create a MySQL data table, you often encounter storage decimals (floating point numbers), such as: Price, weight, height, etc.There are three types of storage solutions prevalent in large companies:1, the data to expand the multiples of 10 to use integer type storage purposes.For example, the price, we often divided into units for storage, that is, to expand the data 100 times times, so that the meta-component sto

Preface: The so-called pit, two meaning, one is the software itself bug, one is the user often make mistakes.Phper often deals with MySQL in daily development. Especially in small and medium-sized applications without layering, phper development should focus on the implementation of SQL statements.To the point, the table statement is built: DROP TABLE IF EXISTS ' test_sql '; CREATE TABLE ' Test_sql ' ( ' id ' int (ten) unsigned not NULL auto_increment COMMENT ' self-increment id

longer as a function, but as a generator generator. Like what def Fab (max): = 0, 0, 1 while n Max: yield b # print B A, B = B, A + b = n + 1 When you execute G = Fab (5), instead of calling a function, you instantiate a generator object. The code inside the FAB function is called when the G object or Next (g) is looped defTest ():#print (' one ') yield1#print (' both ') yield2g=Test () forIinchg:Print(i)#should be the same as the

; importcom.entity.customer; publicclassinsertnewschame{privateconnectionnewconn; Publicinsertnewschame (connectionnewconn) {this.newconn=newconn; }/* * with throwsexception instead of Try/catch, to throw all exceptions to the outer */publicbooleaninsertcustome R (customercustomer,stringid) throwsexception{//account_id\mobile_app_info\ Zhima_score\device_token StringsqlInsert= "Insertintoadm_ Sys_customer (email,idcard_no,login_name,mobile_no,password,real_name) " + "values (?,?,?,?,?,?)";