indexing

问题 I have a Core data model with a 32bit hash value. I need to look up specific hash values quickly. Should I use the indexed property? I have no idea what it does and the documentation is no help (am I looking in the wrong place?) So what does indexed do exactly? 回答1: I would recommend to read this on indexes: http://en.wikipedia.org/wiki/Index_(database). Simply put, a database engine creates a new structure which keeps the indexed column (which corresponds to a property) sorted and a link to

问题 CREATE TABLE `files` ( `did` int(10) unsigned NOT NULL DEFAULT '0', `filename` varbinary(200) NOT NULL, `ext` varbinary(5) DEFAULT NULL, `fsize` double DEFAULT NULL, `filetime` datetime DEFAULT NULL, PRIMARY KEY (`did`,`filename`), KEY `fe` (`filetime`,`ext`), -- This? KEY `ef` (`ext`,`filetime`) -- or This? ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ; There are a million rows in the table. The filetimes are mostly distinct. There are a finite number of ext values. So, filetime has a high

问题 CREATE TABLE `files` ( `did` int(10) unsigned NOT NULL DEFAULT '0', `filename` varbinary(200) NOT NULL, `ext` varbinary(5) DEFAULT NULL, `fsize` double DEFAULT NULL, `filetime` datetime DEFAULT NULL, PRIMARY KEY (`did`,`filename`), KEY `fe` (`filetime`,`ext`), -- This? KEY `ef` (`ext`,`filetime`) -- or This? ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ; There are a million rows in the table. The filetimes are mostly distinct. There are a finite number of ext values. So, filetime has a high

问题 There are many properties in my model that I currently don't need indexed but can imagine I might want indexed at some unknown point in the future. If I explicitly set indexed=False for a property now but change my mind down the road, will Datastore rebuild the entire indices automatically at that point, including for previously written data? Are there any other repercussions for taking this approach? 回答1: No, changing indexed=True to indexed=False (and vice-versa) will only affect entities

问题 My gut tells me there is no good way to achieve this, but, unlike Mr. Stephen Colbert, I'd rather trust a community of developers than my gut... Is there a known way to efficiently implement a "best of both worlds" list, one that provides random access by index and O(1) insertion/removal like a linked list? I foresee two possible outcomes: either "No, this is impossible, for the following obvious reasons..." or "Uh, yes, this has been done; see here, here, and here." 回答1: I don't believe it

问题 Say I have the following model: [Table("Record")] public class RecordModel { [Key] [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)] [Display(Name = "Record Id")] public int RecordId { get; set; } [StringLength(150)] public string Name { get; set; } [Required] [StringLength(15)] public string IMEI { get; set; } } Is it possible to add an index to the IMEI column through using an attribute, data annotation, or something from the model? 回答1: According to this link: Creating Indexes

问题 Since both a Table Scan and a Clustered Index Scan essentially scan all records in the table, why is a Clustered Index Scan supposedly better? As an example - what's the performance difference between the following when there are many records?: declare @temp table( SomeColumn varchar(50) ) insert into @temp select 'SomeVal' select * from @temp ----------------------------- declare @temp table( RowID int not null identity(1,1) primary key, SomeColumn varchar(50) ) insert into @temp select

问题 When adding a unique constraint to a Postgres table does that imply that an index has also been added to that table? Meaning, if I add a UNIQUE constraint on a text column, does that text column now have an index or does an index have to be added separately? 回答1: Yes. UNIQUE constraints are implemented using a btree index in Postgres. Details: How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use? 来源： https://stackoverflow.com/questions/29655439/does-a-postgres

问题 I have a table bsort : CREATE TABLE bsort(a int, data text); Here data may be incomplete. In other words, some tuples may not have data value. And then I build a b-tree index on the table: CREATE INDEX ON bsort USING BTREE(a); Now if I perform this query: SELECT * FROM bsort ORDER BY a; Does PostgreSQL sort tuples with nlogn complexity, or does it get the order directly from the b-tree index? 回答1: For a simple query like this Postgres will use an index scan and retrieve readily sorted tuples

问题 Considering an histogram of shape 100x100x100, I would like to find the 2 highest values a and b, and their indices (a1, a2, a3) and (b1, b2, b3), such as: hist[a1][a2][a3] = a hist[b1][b2][b3] = b We can easily get the highest value with hist.max(), but how can we get the X highest values in a ndarray? I understand that one normally uses np.argmax to retrieve the value indices, but in that case: hist.argmax().shape = () # single value for i in range(3): hist.argmax(i).shape = (100, 100) How