You have three questions: 1. Which __xx__ method has numpy overridden/defined to handle fancy indexing? The indexing operator [] is overridable using __getitem__, __setitem__, and __delitem__. It can be fun to write a simple subclass that offers some introspection: >>> class VerboseList(list): … def __getitem__(self, key): … print(key) … return super().__getitem__(key) … Let’s make an … Read more
Redsandro, You must consider Index Cardinality and Selectivity. 1. Index Cardinality The index cardinality refers to how many possible values there are for a field. The field sex only has two possible values. It has a very low cardinality. Other fields such as names, usernames, phone numbers, emails, etc. will have a more unique value … Read more
If you create a composite primary key, on (x, y, z), PostgreSQL implements this with the help of one UNIQUE multi-column btree index on (x, y, z). In addition, all three columns are NOT NULL (implicitly), which is the main difference between a PRIMARY KEY and a UNIQUE INDEX. Besides obvious restrictions on your data, … Read more
You probably want something like this: >>> a = np.array([True, True, True, False, False]) >>> b = np.array([[1,2,3,4,5], [1,2,3,4,5]]) >>> b[:,a] array([[1, 2, 3], [1, 2, 3]]) Note that for this kind of indexing to work, it needs to be an ndarray, like you were using, not a list, or it’ll interpret the False and … Read more
You probably want something like this: >>> a = np.array([True, True, True, False, False]) >>> b = np.array([[1,2,3,4,5], [1,2,3,4,5]]) >>> b[:,a] array([[1, 2, 3], [1, 2, 3]]) Note that for this kind of indexing to work, it needs to be an ndarray, like you were using, not a list, or it’ll interpret the False and … Read more
Here is my 4 liner: set @exist := (select count(*) from information_schema.statistics where table_name=”table” and index_name=”index” and table_schema = database()); set @sqlstmt := if( @exist > 0, ‘select ”INFO: Index already exists.”’, ‘create index i_index on tablename ( columnname )’); PREPARE stmt FROM @sqlstmt; EXECUTE stmt;
Yes, MySQL can use multiple index for a single query. The optimizer will determine which indexes will benefit the query. You can use EXPLAIN to obtain information about how MySQL executes a statement. You can add or ignore indexes using hints like so: SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX FOR ORDER BY … Read more
In a table without a clustered index (a heap table), data pages are not linked together – so traversing pages requires a lookup into the Index Allocation Map. A clustered table, however, has it’s data pages linked in a doubly linked list – making sequential scans a bit faster. Of course, in exchange, you have … Read more
I don’t know any method to do it. But you could use a list comprehension: >>> [aList[i] for i in myIndices]