大约有 48,000 项符合查询结果（耗时：0.0442秒） [XML]
What are the use cases for selecting CHAR over VARCHAR in SQL?
...omerNumber on a demographic table, or CodeID on a decode table, or perhaps OrderNumber on an order table. By using char, the query engine can more quickly perform the join because it can do straight pointer arithmetic (deterministically) rather than having to move it's pointers a variable amount of...
How to upgrade PostgreSQL from version 9.6 to version 10.1 without losing data?
...is the solution for Ubuntu users
First we have to stop postgresql
sudo /etc/init.d/postgresql stop
Create a new file called /etc/apt/sources.list.d/pgdg.list and add below line
deb http://apt.postgresql.org/pub/repos/apt/ utopic-pgdg main
Follow below commands
wget -q -O - https://www.postg...
What are the differences between git remote prune, git prune, git fetch --prune, etc
...oesn't affect your view of any branches.
git remote prune origin and git fetch --prune both operate on references under refs/remotes/... (I'll refer to these as remote references). It doesn't affect local branches. The git remote version is useful if you only want to remove remote references unde...
Elegant ways to support equivalence (“equality”) in Python classes
...ses, it always calls the method of the subclass operand, regardless of the order of the operands.
So here, if Number is a classic-style class:
n1 == n3 calls n1.__eq__;
n3 == n1 calls n3.__eq__;
n1 != n3 calls n1.__ne__;
n3 != n1 calls n3.__ne__.
And if Number is a new-style class:
both n1 =...
How to sort a list/tuple of lists/tuples by the element at a given index?
...
In order to sort a list of tuples (<word>, <count>), for count in descending order and word in alphabetical order:
data = [
('betty', 1),
('bought', 1),
('a', 1),
('bit', 1),
('of', 1),
('butter', 2),
('but', 1),
('...
What algorithm can be used for packing rectangles of different sizes into the smallest rectangle pos
...ccommodate R, a new level is created.
Bottom-Left (BL) Algorithm
BL first order items by non-increasing width. BL packs the next item as near to the bottom as it will fit and then as close to the left as it can go without overlapping with any packed item. Note that BL is not a level-oriented packin...
Are 2^n and n*2^n in the same time complexity?
...
You will have to go to the formal definition of the big O (O) in order to answer this question.
The definition is that f(x) belongs to O(g(x)) if and only if the limit limsupx → ∞ (f(x)/g(x)) exists i.e. is not infinity. In short this means that there exists a constant M, such that v...
How to do this in Laravel, subquery where in
...uery)
{
$query->select(DB::raw(1))
->from('orders')
->whereRaw('orders.user_id = users.id');
})
->get();
This will produce:
select * from users where id in (
select 1 from orders where orders.user_id = users.id
)
...
What is a “Bitmap heap scan” in a query plan?
... is different from an index scan, where the index is visited row by row in order -- meaning a disk page may get visited multiple times.
Re: the question in your comment... Yep, that's exactly it.
An index scan will go through rows one by one, opening disk pages again and again, as many times as ...
Why is using the rails default_scope often recommend against?
... of default_scope is when you want something to be sorted: default_scope { order(:name) }.
– user2985898
Aug 25 '16 at 9:33
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