indexing

问题 I want make value of TEXT field unique in my MySQL table. After small research I discovered that everybody are discouraging using UNIQUE INDEX on TEXT fields, due to performance issues. What I want to use now is: 1) create another field to contain hash of TEXT value (md5(text_value)) 2) make this hash field UNIQUE index 3) use INSERT IGNORE in queries Is this solution complete, secure and optimal? (found it on SO) Is there a better way of achiving this goal? 回答1: As I was asked in the

问题 This question already has answers here : selecting a range of columns in Python (3 answers) Closed last year . I have yet to find an answer for this anywhere. I am attempting to select columns number 2 and 86:100 . Obviously, I would rather not select them by label. Intuitively I have tried: df_new = df.iloc[:,[2,86:100]] to no avail. What is the most efficient way of selecting these columns? 回答1: You can use np.r_ to combine slices: df = pd.DataFrame(np.random.random((3, 10))) res = df.iloc[

问题 I have time-series DataFrame df that looks like this: time A B C D E 2011-01-04 15:55:00 0.003452 0.005303 0.016632 0.009611 0.000726 ... 2011-01-04 15:56:00 0.004292 0.064709 0.012159 0.020398 0.000272 ... 2011-01-04 15:57:00 0.006617 0.009344 0.018512 0.029696 0.001235 ... 2011-01-04 15:58:00 0.005883 0.048564 0.020213 0.072104 0.001337 ... 2011-01-04 15:59:00 0.008602 0.047318 0.024568 0.051225 0.002775 ... I want to add a multiindex header to categorize each columns. So for instance, I

问题 I have been thinking about two questions. Couldn't find any resources on the internet about this. How do dbms handle it ? Or do they ? Especially Oracle. Before the questions, here is an example: Say I have a master table "MASTER" and slave table "SLAVE". Master table has an "ID" column which is the primary key and index is created by Oracle.Slave table has the foreign key "MASTER_ID" which refers to master table and "SLAVE_NO". These two together is the primary key of slave table, which is

问题 Background I have many (thousands!) of data files with a standard field based format (think tab-delimited, same fields in every line, in every file). I'm debating various ways of making this data available / searchable. (Some options include RDBMS, NoSQL stuff, using the grep/awk and friends, etc.). Proposal In particular, one idea that appeals to me is "indexing" the files in some way. Since these files are read-only (and static), I was imagining some persistent files containing binary trees

问题 If I have a vector of: [4,5,6,7,11,12,13,14,21,22,23] How can I, without a loop , extract the start/end values of all consecutive number blocks i.e. the desired result for the above vector would be a 2-column vector: b = 4 7 11 14 21 23 回答1: Easy: a = [4,5,6,7,11,12,13,14,21,22,23]; b = reshape(a(sort([find(a - circshift(a,[0,1]) ~= 1),find(a - circshift(a,[0,-1]) ~= -1)])),2,[])' Output: b = 4 7 11 14 21 23 回答2: Another approach: x = [4,5,6,7,11,12,13,14,21,22,23]; x = x(:); ind = find([1;

问题 I just started programming in Elm and am stuck at something: I would like to have a method that can update fields of elements in a list at a certain index. My signature would look like this: updateElement : List (ID, Task) -> Int -> List (ID, Task) with: type alias Task = { description : String, focus : Bool} In this case I would like to set the boolean (focus) of the task at the index given to true and all the others tasks in the list to false. I already tried with arrays in Elm but then I

问题 Is it possible to add a constraint like ALTER TABLE `t1` ADD UNIQUE(`col1`, `col2`); without creating an index? The index wouldn't be used for any queries so it would be a waste of space. It wouldn't be a problem if inserts and updates would be way slower, because the table doesn't get updated very often. 回答1: No, this is not possible. A UNIQUE constraint contains an index definition and I barely imagine how it might be implemented without creating an index (in DBMS terms). You should realize

问题 emperor <- rbind(cbind('Augustus','Tiberius'),cbind('Caligula','Claudius')) How do I return the row and column numbers of all the cells that contain the sequence 'us', i.e. [1,1], [1,2], [2,2]? 回答1: We could use grepl to get a vector of logical index, convert to a matrix of the same dimension as the original matrix ('emperor') and wrap with which with arr.ind=TRUE . which(matrix(grepl('us', emperor), ncol=ncol(emperor)), arr.ind=TRUE) # row col #[1,] 1 1 #[2,] 1 2 #[3,] 2 2 Or another way to

问题 We're having a problem where a mysqldump script is spending 90% of it's time populating a small handful of the tables it deals with. Eliminating FK's and indexes eliminates the speed problem, but is not an acceptable solution. The dump script does have: /*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */; /*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */; Can we expect any different behavior from ALTER TABLE foo DISABLE KEYS ? Also, is disable