indexing

问题 What is the difference between primary , unique and foreign key constraints , and indexes ? I work on Oracle 10g and SQL Server 2008 回答1: Primary Key and Unique Key are Entity integrity constraints Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered. Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle one null is not equal to

问题 What is the difference between primary , unique and foreign key constraints , and indexes ? I work on Oracle 10g and SQL Server 2008 回答1: Primary Key and Unique Key are Entity integrity constraints Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered. Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle one null is not equal to

问题 I'd like to ask fellow SO'ers for their opinions regarding best of breed data structures to be used for indexing time-series (aka column-wise data, aka flat linear). Two basic types of time-series exist based on the sampling/discretisation characteristic: Regular discretisation (Every sample is taken with a common frequency) Irregular discretisation(Samples are taken at arbitary time-points) Queries that will be required: All values in the time range [t0,t1] All values in the time range [t0

问题 If we have a known value in a column, how can we get its index-value? For example: In [148]: a = pd.DataFrame(np.arange(10).reshape(5,2),columns=['c1','c2']) In [149]: a Out[149]: c1 c2 0 0 1 1 2 3 2 4 5 ........ As we know, we can get a value by the index corresponding to it, like this. In [151]: a.ix[0,1] In [152]: a.c2[0] In [154]: a.c2.ix[0] <-- use index Out[151]: 1 Out[152]: 1 Out[154]: 1 <-- get value But how to get the index by value? 回答1: There might be more than one index map to

问题 In software engineering we create indexes all the time (e.g., in databases) but I also hear a lot of people talk about inverted indices. Is there something fundamentally different between the two? They sound like the same thing. 回答1: One common use is "...to allow fast full-text searching." The two types denote directionality . One takes you forward through the index, and the other takes you backward (the inverse) through the index. That's it. There's no mystery to uncover here. Otherwise the

问题 I have a View Controller with a button, an index, and a function thats called when the button is pressed, here is an example of the code: View Controller: func buttonPressed(){ index++ } Then i have a class in which i want to access and print the index from the View Controller Class: print("Index is \(ViewController().index)") Obviously this doesn't work, does anyone know how I can access this? I can't really instantiate it because its a ViewController as well as it will not be the same index

问题 I have a query that uses compound index with sort on "_id". The compound index has "_id" at the end of the index and it works fine until I add a $gt clause to my query. i.e, Initial query db.colletion.find({"field1": "blabla", "field2":"blabla"}).sort({_id:1} Subsequent queries db.colletion.find({"field1": "blabla", "field2":"blabla", _id:{$gt:ObjetId('...')}}).sort({_id:1} what I am noticing is that there are times when my compound index is not used. Instead, Mongo uses the default

问题 I couldn't find anything matching my problem, so hopefully this wasn't already mentioned somewhere and I'm just too stupid to find it. thelist = [] a = [0] for i in range(5): thelist.append(a) print(thelist) At this point, the program returns [[0], [0], [0], [0], [0]] thelist[0].append(1) print(thelist) After appending this I would expect it to return the same but with the first element modified, like this: [[0, 1], [0], [0], [0], [0]] What actually happens, is that every element was modified

问题 I have a Json document in cloudant as: { "_id": "3-f812228f45b5f4e4962505561953ew245", "_rev": "3-f812228f45b5f4e496250556195372b2", "wiki_page": "http://en.wikipedia.org/wiki/African_lion", "name": "african lion", "class": "mammal", "diet": "herbivore" } I want to make a search index that can search this document when I input queries as "afrian lion" or "lion african",... I make a function that can return all cases of permutation in "doc.name" for indexing (This function works well and it

问题 I have indexed a couple of documents using solr, now when I perform a search using the admin interface, it returns search results in the XML format. I am trying to figure out how can I associate a document that I have indexed example: test.pdf with the results that I receive and then serve that document to my user ? Will solr return to me a unique ID of the document that I index, so that after indexing a document I can store the document along with that UID in my database somewhere and then