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MySQL error: key specification without a key length
...d, and the trick is to choose a number N that’s long enough to give good selectivity, but short enough to save space. The prefix should be long enough to make the index nearly as useful as it would be if you’d indexed the whole column.
Before we go further let us define some important terms. In...
How to copy a row and insert in same table with a autoincrement field in MySQL?
...
Use INSERT ... SELECT:
insert into your_table (c1, c2, ...)
select c1, c2, ...
from your_table
where id = 1
where c1, c2, ... are all the columns except id. If you want to explicitly insert with an id of 2 then include that in your INSER...
Where and how is the _ViewStart.cshtml layout file linked?
...wStart.cshtml allows us to write code, we
can optionally make our Layout selection logic richer than just a
basic property set. For example: we could vary the Layout template
that we use depending on what type of device is accessing the site –
and have a phone or tablet optimized layout f...
How to create relationships in MySQL
...s defined as the default storage engine,can check using command (mysql> SELECT @@default_storage_engine;)
– Arun
Feb 18 at 4:00
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Why does MYSQL higher LIMIT offset slow the query down?
...h more than 16 million records [2GB in size]. The higher LIMIT offset with SELECT, the slower the query becomes, when using ORDER BY *primary_key*
...
What is the “N+1 selects problem” in ORM (Object-Relational Mapping)?
The "N+1 selects problem" is generally stated as a problem in Object-Relational mapping (ORM) discussions, and I understand that it has something to do with having to make a lot of database queries for something that seems simple in the object world.
...
Remove duplicate rows in MySQL
...ate temporary table tmpTable (id int);
insert into tmpTable
(id)
select id
from YourTable yt
where exists
(
select *
from YourTabe yt2
where yt2.title = yt.title
and yt2.company = yt.company
and yt2.site_id = yt.si...
How to find the kth smallest element in the union of two sorted arrays?
...ithmic. (Here we are getting rid of half). In order to do that, we need to select one array whose one of the halves we can safely ignore. How do we do that? By confidently eliminating the half we know for sure is not going to have the kth element.
– lambdapilgrim
...
How to find Unused Amazon EC2 Security groups
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If you select all of your security groups in the EC2 console, then press actions -> Delete Security Groups, a popup will appear telling you that you cannot delete security groups that are attached to instances, other security gro...
搭建高可用mongodb集群（三）—— 深入副本集内部机制 - 大数据 & AI - 清...
...着副本集的问题来看吧！
副本集故障转移，主节点是如何选举的？能否手动干涉下架某一台主节点。
官方说副本集数量最好是奇数，为什么？
mongodb副本集是如何同步的？如果同步不及时会出现什么情况？会不会出现不一致...