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What is the difference between mysql unique and non-unique index in terms of performance? Let us say I want to make an index on a combo of 2 columns, and the combination is unique, but I create a non-unique index. Will that have any significant effect...
Started by Sanjay on
, 3 posts
by 3 people.
Answer Snippets (Read the full thread at stackoverflow):
As far as a primary key is concerned, probably quite, because it means that it won't... .
On a non-unique index that just happens to be unique and a unique index? I'm not sure, but I'd be the number of rows, for a unique index).
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Hi,
I have a table with a column of unique string values. The max length of the string value is 255 char. I want to generate a unique id with the string value as input. In other words I am looking for a compact representation for a string. The unique ...
Started by pkrish on
, 8 posts
by 8 people.
Answer Snippets (Read the full thread at stackoverflow):
How unique is "unique"? Using any java.security.MessageDigest....
If your database requires that the column contain unique the original, a hash function such as MD5 is what you're looking for.
Constraints on it than "be unique".
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Is there a better way than what is proposed here to create a one-to-one mapping between an table view cell's NSIndexPath and a unique NSUInteger or int , in order to create a unique tag property value for a widget in the cell (a UIProgressView )?
The ...
Started by Alex Reynolds on
, 3 posts
by 3 people.
Answer Snippets (Read the full thread at stackoverflow):
(if ....
If you have a table with 2 sections, each with 3 billion rows, then there's no way to map those 6 billion potential NSIndexPaths into the 4 billion 32-bit NSIntegers .
No, there really isn't a way that is guaranteed to work for any number of rows .
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Ask your Facebook Friends
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I'm working on an application that will be implementing a hex value as a business key (in addition to an auto increment field as primary key) similar to the URL id seen in Gmail. I will be adding a unique constraint to the column and was originally thinking...
Started by tgm on
, 4 posts
by 4 people.
Answer Snippets (Read the full thread at stackoverflow):
A potentially larger problem ....
Is your hex value a GUID? Although I used to worry about the performance of such long items as indexes, I have found that on modern databases the performance difference on even millions of records is fairly insignificant .
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My need is to have items in kind of Collections.Generic.Dictionary where I can get a struct by it's id as a key. Then I have need to fetch many structs, say 1% or less of all items, by another field. Like a cursor by an non-unique index. With Dictionary...
Started by matti on
, 4 posts
by 4 people.
Answer Snippets (Read the full thread at stackoverflow):
You had:
class Employee { int DeptID; // A non-unique field we want to index; }
I don't believe there is a built in Dictionary like collection accepting non-unique TKey values, but you.
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Is a GUID unique 100% of the time?
Will it stay unique over multiple threads?
Started by David Basarab on
, 13 posts
by 13 people.
Answer Snippets (Read the full thread at stackoverflow):
It is based on both hardware....
Yes, a GUID should always be unique.
From Wikipedia.
While each generated GUID is not guaranteed to be unique, the total number of unique keys (2^128 then have 6.8×10^15 universally unique GUIDs.
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I want to find a replacement. I just don't care for the stuff that much, I don't hate it, but I find it dirty, smokey and I don't like the way it smells. I think it may be too fast burn wise for what I want to do as well.
There is no denying that it is...
Started by gofastman on
, 15 posts
by 14 people.
Answer Snippets (Read the full thread at gunloads):
5 behaves sort of like Unique in several cartridges
Sent from my Lumia 900 using Board Express Universal Clays....
Accurate No.
I like Herco, but you might find the things that inconvience you about Unique to exist it would not work.
For grain.
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Does it slow down the query time to use a lot of unique indexes? i dont have that many im just curious, i think i have heard this some where
id (primary auto_increment) username (unique) password salt (unique) email (unique)
Started by tomek on
, 6 posts
by 6 people.
Answer Snippets (Read the full thread at stackoverflow):
In general , unique indexes time you add a unique key....
If you need a unique index to guarantee consistency in your application then you should usually add, but not necessarily by a lot - this will depend on the table size as well.
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Guys,
I wanted a number that would remain unique for a day (24 hours). Following is the code that I came up with; I was wondering about its fallacies/possible risks; 'I believe' this guarantees a 12 digit unique number for a day atleast.
Logic is to get...
Started by KMan on
, 8 posts
by 8 people.
Answer Snippets (Read the full thread at stackoverflow):
//Guarantees a unique number for a calendar date, //that in a single milli second band ....
Out of curiosity, is there a reason]) { //Logic: Add HHMMSSmmm with 3 bytes counter .
Numbers will definitely not be unique across machines, however.
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What is the difference between using xsd:unique and xsd:key?
Started by Joepie on
, 4 posts
by 4 people.
Answer Snippets (Read the full thread at stackoverflow):
unique and id
unique....
Key field is like primarykey..
It is similar to database.
Hope this helps...
Xsd:key is unique AND is used for a collection of elements a key within a scope xsd:unique is just unique within a scope.
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