indexing

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Specifying an index (non unique key) using JPA Is there a way to define index on enitity column, to improve searches performance? I saw that hibernate gives @Index and @IndexColumn, but I am looking for JPA way to do it. thanks Here is an example of my entity, I need to index a name column @Entity @Table(name = "MY_TABLE") public class MyEntity { private long id; private String name; private String sourceInfo; .

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Specifying an index (non unique key) using JPA Is there a way to define index on enitity column, to improve searches performance? I saw that hibernate gives @Index and @IndexColumn, but I am looking for JPA way to do it. thanks Here is an example of my entity, I need to index a name column @Entity @Table(name = "MY_TABLE") public class MyEntity { private long id; private String name; private String sourceInfo; .

问题 I'm trying to understand what is better when defining multi-column indexes: Putting the most selective column first (higher cardinality, for speed?); or Putting the less selective column first (Lower cardinality, for index compression?) Or maybe it depends if I'm optimizing for speed or space? 回答1: The order of the columns should match the order in which the columns are queried later or MySQL will not use them. This is the question you should really think about. Read more here. UPDATE: For

问题 I am trying to search through a Pandas Dataframe to find where it has a missing entry or a NaN entry. Here is a dataframe that I am working with: cl_id a c d e A1 A2 A3 0 1 -0.419279 0.843832 -0.530827 text76 1.537177 -0.271042 1 2 0.581566 2.257544 0.440485 dafN_6 0.144228 2.362259 2 3 -1.259333 1.074986 1.834653 system 1.100353 3 4 -1.279785 0.272977 0.197011 Fifty -0.031721 1.434273 4 5 0.578348 0.595515 0.553483 channel 0.640708 0.649132 5 6 -1.549588 -0.198588 0.373476 audio -0.508501 6

问题 I know that I can reset the indices like so df.reset_index(inplace=True) but this will start the index from 0 . I want to start it from 1 . How do I do that without creating any extra columns and by keeping the index/reset_index functionality and options? I do not want to create a new dataframe, so inplace=True should still apply. 回答1: Just assign directly a new index array: df.index = np.arange(1, len(df) + 1) Example: In [151]: df = pd.DataFrame({'a':np.random.randn(5)}) df Out[151]: a 0 0

问题 A. How do I index "nested" and all of it's values? B. How do I index valuetwo? { id: 00000, attrs: { nested:{ value: value1, valuetwo: value2, } } } I've looked here: http://www.mongodb.org/display/DOCS/Indexes, and the docs to my knowledge, aren't clear about indexing things that aren't nested. 回答1: You'd create them just as if you were creating an index on a top level field: db.collection.ensureIndex({"attrs.nested.value": 1}) You do need to explicitly create indexes on each field. 回答2: A.

问题 I've got (for example) an index: CREATE INDEX someIndex ON orders (customer, date); Does this index only accelerate queries where customer and date are used or does it accelerate queries for a single-column like this too? SELECT * FROM orders WHERE customer > 33; I'm using SQLite. If the answer is yes, why is it possible to create more than one index per table? Yet another question: How much faster is a combined index compared with two separat indexes when you use both columns in a query? 回答1

问题 I am wondering if there is any disadvantage on defining a column of type nvarchar(max) instead of giving it a (smaller) maximum size. I read somewhere that if the column value has more than 4?KB the remaining data will be added to an "overflow" area, which is ok. I'm creating a table where most of the time the text will be of a few lines, but I was wondering if there's any advantage in setting a lower limit and then adding a validation to avoid breaking that limit. Is there any restriction on

问题 This question already has answers here : How to get index using LINQ? [duplicate] (7 answers) Closed 5 years ago . I am new to Linq. I have a Customers table.ID,FullName,Organization,Location being the columns. I have a query in Sqlite returning me 2500 records of customers. I have to find the index of the customer where ID=150 for example from this result set. Its a List of Customers. The result set of the query is ordered by organization. I tried with FindIndex and IndexOf but getting

问题 Currently our database has size 10 GB and is growing by around 3 GB per month. Often I hear that one should from time to time rebuild the indexes, to improve the query execution time. So how often should I rebuild the indexes in the given scenario? 回答1: There's a general consensus that you should reorganize ("defragment") your indices as soon as index fragmentation reaches more than 5 (sometimes 10%), and you should rebuild them completely when it goes beyond 30% (at least that's the numbers