sql - What is a multiple key index? -



sql - What is a multiple key index? -

add_index :microposts, [:user_id, :created_at]

i going through michael hartl's railstutorial , noticed he's using called multiple key index. know means active record uses both keys @ same time, i'm not sure advantages , disadvantages of using multiple key indexes are.

if can give reply appreciate it.

any index can give benefit allowing query narrow downwards set of rows examine.

a multi-column index can help when query includes conditions on multiple columns.

for example:

select * mytable user_id = 123 , created_at > '2013-02-01'

the multi-column index narrows downwards subset of rows associated user_id 123, within subset, farther narrows downwards selection recent created_at value.

without sec column in index, rdbms have load all rows user_id 123 memory before determine if pass criteria.

for more information, see presentation how design indexes, really.

sql ruby-on-rails

Comments

Post a Comment

Popular posts from this blog

web services - java.lang.NoClassDefFoundError: Could not initialize class net.sf.cglib.proxy.Enhancer -

web services - java.lang.NoClassDefFoundError: Could not initialize class net.sf.cglib.proxy.Enhancer - when trying request rest service @ time of response getting exception.. feb 21, 2013 2:34:49 pm com.sun.jersey.spi.container.containerresponse mapmappablecontainerexception severe: exception contained within mappablecontainerexception not mapped response, re-throwing http container java.lang.noclassdeffounderror: not initialize class net.sf.cglib.proxy.enhancer @ org.hibernate.proxy.pojo.cglib.cgliblazyinitializer.getproxyfactory(cgliblazyinitializer.java:117) @ org.hibernate.proxy.pojo.cglib.cglibproxyfactory.postinstantiate(cglibproxyfactory.java:43) @ org.hibernate.tuple.entity.pojoentitytuplizer.buildproxyfactory(pojoentitytuplizer.java:162) @ org.hibernate.tuple.entity.abstractentitytuplizer.<init>(abstractentitytuplizer.java:135) @ org.hibernate.tuple.entity.pojoentitytuplizer.<init>(pojoentitytuplizer.java:55) @ org.hibernate.tuple.e...
Read more

Accessing MATLAB's unicode strings from C -

Accessing MATLAB's unicode strings from C - how can access underlying unicode info of matlab strings through matlab engine or mex c interfaces? here's example. let's set unicode characters in utf-8 encoded file test.txt, read as fid=fopen('test.txt','r','l','utf-8'); s=fscanf(fid, '%s') in matlab. now if first feature('defaultcharacterset', 'utf-8') , c engevalstring(ep, "s") , output text file utf-8. proves matlab stores unicode internally. if mxarraytostring(enggetvariable(ep, "s")) , unicode2native(s, 'latin-1') give me in matlab: non-latin-1 characters replaced character code 26. need getting access underlying unicode info c string in unicode format (utf-8, utf-16, etc.), , preserving non-latin-1 characters. is possible? my platform os x, matlab r2012b. addendum: documentation explicitly states "[mxarraytostring()] supports multibyte encoded characters...
Read more

javascript - mongodb won't find my schema method in nested container -

javascript - mongodb won't find my schema method in nested container - i trying access method of schema stored within mixed container. here situation : have cases model can many different things, have schema each of these things stored in "casecontent" mixed property. var caseschema = mongoose.schema({ casecontent : {}, object : {type:string, default : "null"}, collision : {type : boolean, default : false} }); the casecontent property filled model of 1 of schemas, 1 exemple : var treeschema = new mongoose.schema({ applecount : {type : number, default : 3} }); treeschema.methods.dostuff = function (data) { console.log('hey, listen'); homecoming true; }; then, want utilize method of schema original container : caseschema.methods.dostuff = function (data) { if (this.casecontent.dostuff !== undefined) { this.casecontent.dostuff(); console.log('it worked'); } else { ...
Read more