indexing - Why most SQL databases allow defining the same index twice? -

-

Why do most SQL databases allow to define a single index (or constraint) twice?

For example in MySQL I can: 

  Make Table Tester (id VARCHAR (10) Faucet, Primary Key (ID)); Optional table testkey plus key (id); Optional table testkey plus key (id); Create Table Tester; Create tab 'Tad kaki' (`id` varchar (10) tap, primary key (` id`), key `id` (` id`), key 'id_2` (`id`))   
 I do not see any use to get the same index or hurdle twice. And I would like the SQL database not allow me to do this. 
 
 I do not even see the names or names of the barriers, because I can refer to them for reference to remove them.

Many reasons come to mind in the case of a database product that supports several index types, It is possible that you want a combination of indexed fields of the same field or multiple times, depending on the different ones used with each index. For example, some (probably the most) database products have a tree-structured index for both straightforward lookups (for example KEY_FIELD = 1) and border scans (such as KEY_FIELD> 0 and KEY_FIELD <5) that's good. In addition, some (but not all, of course) database products also support a handy index type, which is useful only for direct lookup, but which is very fast (like working for a comparison like KEY_FIELD = 1 , But it can not be used to compare a limit) If you need fast looking time but still need to provide it for comparison then it can be useful to create both a tree-structured index and a palm indicator.

Some database products have several primary key constraints on a table, however, it requires more effort on the part of the database vendor to prevent all possible duplicates, because they think it may be appropriate. In the case of an open source database, major developers believe that if a given attribute is a great deal for a user, then that user should have the facility to send a code patch to it. Wanted. Open source "I use your open source product, so you are now my slave and I should apply every feature as usual."

In the end, I think it is fair to say that a product for use by software developers should expect it to take proper care during the use of the given product.


Comments

Post a Comment

Popular posts from this blog

c# - sqlDecimal to decimal clr stored procedure Unable to cast object of type 'System.Data.SqlTypes.SqlDecimal' to type 'System.IConvertible' -

-
I'm new to this I'm writing a clr. stored procedure that can be used to get values ​​used in a calculation Call for another stored procedure for Gives an archived methodology int (I think SqlInt32? Value of the table's value is int) which is then necessary to convert to decimal. Here I have the code: Public static integral mood (SQL MDMMYLUE, escalistering AMY type, out of SQLDCM Result) {aResult = 0; Try {SqlConnection conn = New SqlConnection ("Reference Connection = True"); SqlCommand cmd = New SqlCommand ("sp_GetType", conn); Cmd.Parameters.AddWithValue ("@ myValueType", aMyValueType); Cmd.CommandType = CommandType.StoredProcedure; Conn.Open (); Object type = CMD. Exclamation Asser (); Conn.Close (); Decimal type FORCalculation = Convert. ToDecimal (type); Decimal value = convert toDecimal (aMyValue); // ** calculation calc = new calculation (); Decimal result = calorie.docalulation (typefarculation, value); I get an exception...
Read more

Calling GetGUIThreadInfo from Outlook VBA -

-
I need to find the active window'm writing a VBA macro in MS Outlook, which is a specific process. The goal of this is to get the active window / document name even if its original application has more than one document interface. User processor with GetCurrentThreadId is active, I am using GetGUIThreadInfo to get .dll. Here is the correct code: private type RECT as as Left LongPtr top LongPtr right LongPtr down LongPtr termination of private type GUITHREADINFO cbSize as LongPtr flags LongPtr hwndActive LongPtr hwndFocus LongPtr hwndCapture LongPtr as as as as as reset as LongPtr rcCaret as LongPtr hwndCreate as LongPtr hwndMoveSize as HwndMenuOwner end of Private Equity PtrSafe function GetWindowThreadProcessId Lib "user32" (_BVV hnw LongPtr as LongPtr Nig Mr. announced as PtrSafe function GetGUIThreadInfo Lib "user32" _ (ByVal dwthreadid as LongPtr, _ lpguithreadinfo as GUITHREADINFO) LongPtr Sub MyFunction () as dim strWindowTitle string = space as s...
Read more

Obfuscating Python code? -

-
I'm looking for a way to hide my Python source code. print "Hello World!" How can I encode this example so that it is not human-readable? I have been asked to use Base 64 but I'm not sure how. itemprop = "text"> It is limited only, first level ambiguous resolution, but has built-in: byte-code in Python Is a compiler for: dragon-o-m py_compile & lt; Your program.py & gt; creates a .pyo file with a byte-code, and where the docstrings are removed, etc. You can rename .pyo with the .py extension, and python & lt; Your program.py & gt; runs as your program, but it does not have your source code. PS : If your program blur modules like this, you need to rename them with the .pyc suffix (I Not sure it will not break a day), or you can work with .pyo and run them with python -o ... .pyo ( Import should work). This will allow dragon to search your modules (otherwise, Python looks for the .py module).
Read more