sql server 2008 - SQL Basic Concurrency -

- 

insert t1 (xfield) select 7 xfield (select count(*) t1) = 0

basically, if million users run @ same time, there ever end being more 1 row in t1?

thread a: select count(*) t1: 0 thread b: select count(*) t1: 0 thread a: insert t1... thread b: insert t1...

or guaranteed never happen, because it's 1 statement?

if isn't safe, this?

table t2 (gonorth , gosouth must never both 1):

id      gonorth   gosouth 1       0         0

then happens:

user a: update t2 set gonorth = 1 gosouth = 0 user b: update t2 set gosouth = 1 gonorth = 0  thread a: find rows gosouth = 0 thread b: find rows gonorth = 0 thread a: found row gosouth = 0 thread b: found row gonorth = 0 thread a: setting gonorth = 1 located row thread b: setting gosouth = 1 located row

and result:

id      gonorth   gosouth 1       1         1

what rules can happen @ same time , can't?

my database engine "microsoft sql server 2008 r2 (sp2)".

no. it's written single statement, , therefore rules of sql provide entire statement atomic.

however, there caveats here. first of all, mean creating quite large number of locks, point table becomes unavailable until query completed. in other words, in order guarantee safety, you'd throwing away concurrency. other caveat holds default isolation level. weaker isolation level may allow query run without creating appropriate locks. weak isolation level might allow ignore locks.


Comments

Post a Comment

Popular posts from this blog

java - Jmockit String final length method mocking Issue -

-
using jmockit 1.2 jar, trying mock string's length method getting unexpected invocation exception: failed: test java.lang.illegalstateexception: missing invocation mocked type @ point; please make sure such invocations appear after declaration of suitable mock field or parameter @ stringdemo.testa$1.<init>(testa.java:17) @ stringdemo.testa.test(testa.java:13) i using testng: @test public void test() throws exception { new expectations() { @mocked("length") string astring; { astring.length(); result = 2; } }; system.out.println(a.showa("test")); } } actual class a: public class { public static int showa(string str){ int a= str.length(); return a; } } this wrong way of recording expected results. shouldn't mock , record string's length() method, record showa() instead. here solution @suppresswarnings("unused&qu
Read more

asp.net - Razor Page Hosted on IIS 6 Fails Every Morning -

-
i have simple page in intranet uses razor/asp fetch single record table , display it, plus few graphic, dashboard display. meaning, no user intervention other first time open ie internal url. the problem having every morning ie displays "page not found or network error" message. refresh page, , it's stuck , doesn't display anything. try different pc, open ie internal url , it's stuck... ...until following: login server hosting page run inetmgr go web sites, etc right click on page that's giving me problem , select browse at moment, error message: server error in '/' application. -------------------------------------------------------------------------------- type of page not served. description: type of page have requested not served because has been explicitly forbidden. extension '.cshtml' may incorrect. please review url below , make sure spelled correctly. requested url: /application/dashboard.cshtml ---------------
Read more

c++ - wxwidget compiling on windows command prompt -

-
can please guide me on method,as how compile wxwidget using windows command prompt or msys. [i can't seem find on wxwiki/wxwidget official book/anywhere else] i have compiled wxwidget instructions have provided (using msys). from searching on internet seems 1 can (maybe?) through use of makefile (is correct?) if yes: directory & how should link wx libraries in makefile if no: way besides makefile? i know can use ide's code::block , make life simpler prefer compile using command prompt/msys. thanks in advance.
Read more