indexing

问题 I've got a PostgreSQL array of strings as a column in a table. I created an index using the GIN method. But ANY queries won't use the index (instead, they're doing a sequential scan of the whole table with a filter). What am I missing? Here's my migration: class CreateDocuments < ActiveRecord::Migration def up create_table :documents do |t| t.string :title t.string :tags, array: true, default: [] t.timestamps end add_index :documents, :tags, using: 'gin' (1..100000).each do |i| tags = [] tags

问题 I have such a function lnn1c(ii, j, n, n1) which takes indexes ii and jj as arguments where Kdk1 and Wdg are some arrays, wg(n) is another function kinda alpha*(n-3) and Gdg is a symbolic variable. function lnn1c=lnn1c(ii, j, n, n1) syms k1Vzdg global Gdg Wdg Kdk1 lnn1c=Gdg-i*(-(Wdg(ii)-Wdg(j))+(wg(n)-wg(n1))+... (Kdk1(ii)-Kdk1(j))*k1Vzdg); end I wanna perform in my script summation of expression lnn1c(ii, j, n, n1) over indexes ii and j from 1 up to 4. I tried such code syms ii jj n n1 sum

问题 I've installed solr 4.6.0 and follow the tutorial available at Solr's home page. Everything was fine, untill I need to do a real job that I'm about to do. I have to get a fast access to wikipedia content and I was advised to use Solr. Well, I was trying to follow the example in the link http://wiki.apache.org/solr/DataImportHandler#Example:_Indexing_wikipedia, but I couldn't get the example. I am newbie, and I don't know what means data_config.xml! <dataConfig> <dataSource type=

问题 I have a table watchlist containing today almost 3Mil records. mysql> select count(*) from watchlist; +----------+ | count(*) | +----------+ | 2957994 | +----------+ It is used as a log to record product-page-views on a large e-commerce site (50,000+ products). It records the productID of the viewed product, the IP address and USER_AGENT of the viewer. And a timestamp of when it happens: mysql> show columns from watchlist; +-----------+--------------+------+-----+-------------------+-------+

问题 I have a table with columns like word, A_, E_, U_ .. these columns with X_ are tinyints having the value of how many times the specific letter exists in the word (to later help optimize the wildcard search query). There is totally 252k rows. If i make search like WHERE u_ > 0 i get 60k rows. But if i do the explain of that select, it says there is 225k rows to go through and no index possible. Why? Column was added as index. Why it doesn't say there is 60k rows to go through and that possible

问题 IMO, please correct me... the leaf of clustered index contains the real table row, so full clustered index, with intermediate leaves, contain much more data than the full table(?) Why/when/how is ever whole clustered index scan chosen over the full table scan? How is clustered index on CUSTOMER_ID column used in SELECT query which does not contain it in either SELECT list or in WHERE condition [1]? Update: Should I understand that full clustered scan is faster than full table scan because

问题 I'm working on drawing a terrain in WebGL. The problem is that I'm only using 4 vertices to draw a single quad by using indices to share vertices. So I can't upload unique baricentric coordinates for each vertex, because it's shared. Here's a picture that shows the problem more clearly. There's no barycentric coordinate that I can use for the question mark. (0,1,0) is used top left, (0,0,1) is used above and (1,0,0) is used to the left. So there's absolutely no way I can do this when I'm

问题 I have an index on a nullable column and I want to select all it's values like this: SELECT e.ename FROM emp e; In the explain plan I see a FULL TABLE SCAN (even a hint didn't help) SELECT e.ename FROM emp e WHERE e.ename = 'gdoron'; Does use the index... I googled and found out there are no null entries in indexes, thus the first query can't use the index. My question is simple: why there aren't null entries in indexes? 回答1: By default, relational databases ignore NULL values (because the

问题 I have recently completely re-written a large project. In doing so, I have consolidated great number of random MySQL queries. I do remember that over the course of developing the previous codebase, I created indexes on a whim, and I'm sure there are a great number that aren't used anymore. Is there a way to monitor MySQL's index usage to determine which indexes are being used, and which ones are not? 回答1: I don't think this information is available in a stock MySQL installation. Percona makes

问题 I was wondering if there is an equivalent way to add a row to a Series or DataFrame with a MultiIndex as there is with a single index, i.e. using .ix or .loc? I thought the natural way would be something like row_to_add = pd.MultiIndex.from_tuples() df.ix[row_to_add] = my_row but that raises a KeyError. I know I can use .append(), but I would find it much neater to use .ix[] or .loc[]. here an example: >>> df = pd.DataFrame({'Time': [dt.datetime(2013,2,3,9,0,1), dt.datetime(2013,2,3,9,0,1)],