composite-index | 易学教程

EF Migrations drops index when adding compsite index

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问题 I noticed EF removed an index on a foreign key when I added a composite index with the foreign key. So I need to understand composite indexes better :) I added the composite index using this answer and generated my EF code first migration file. Adding composite index: this.Property(x => x.Name) .HasUniqueIndexAnnotation("IX_UniqueNamePerKey", 0); this.Property(x => x.TeacherId) .HasUniqueIndexAnnotation("IX_UniqueNamePerKey", 1); Migration file: public partial class CompositeIndex :

Index on Composite attributes

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问题 When we create an index on an attribute a tree is created for this attribute. But what happens when we create an index with composite attributes? Two trees are created? Both are part of the same tree? What? 回答1: It concats the attributes in the same order as you have mentioned. It for the same reason, if you have an composite index on columns a,b,c in the same order, the index will be useful only if the left columns are searched WHERE a=4 ## uses index WHERE a=4 and b=10 ## uses index WHERE b

Do mysql composite indexes make some other indexes completely redundant?

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问题 If I have an a composite index on (a, b) I understand that queries only concerned with 'a' will still use the composite index (but not queries concerned with 'b') My question is whether there is any valid reason to have a single-column index on 'a' if I have the (a, b) index? What I've read has seemed vague as to whether the (a,b) index was a complete substitute for a, or merely a "better than nothing" index. This assumes that I do filtering by both a and a,b. I have a table with way too many

Does wildcard in left-most column of composite index mean remaining columns in index aren't used in index lookup (MySQL)?

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问题 Imagine you have a primary composite index of last_name,first_name . Then you performed a search of WHERE first_name LIKE 'joh%' AND last_name LIKE 'smi%' . Does the wildcard used in the last_name condition mean that the first_name condition will not be used in further helping MySQL find indexes? In other words, by putting a wildcard on the last_name condition MySQL will only do a partial index lookup (and ignores conditions given in the columns that are to the right of last_name)? Further

How to avoid a filesort on a mysql composite index for a prefix query with an order by statement?

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问题 I have a 1 GB mysql table with three colums (german bigrams): create table sortedindex (source varchar(60),target varchar(60),score float) engine=myisam character set utf8 collate utf8_bin; I also created a composite index: create index sortedstd_ix on sortedindex (source(60), target(60), score); additionally I compressed the table and made it read only and sorted the index using: myisamchk --keys-used=0 -rq sortedindex myisampack sortedindex myisamchk -rq sortedindex --sort_buffer=3G --sort

Separate Join clause in a Composite Index

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问题 Would having a Composite Index be beneficial for something like this: SELECT * FROM a INNER JOIN b ON(a.id=b.id) INNER JOIN c ON(a.bar=c.id) INNER JOIN d ON(a.foo=d.id) Index would be: (a.id, a.bar, a.foo) 回答1: Only the leading edge of the index would be used ( a.id ), so only the INNER JOIN to b would benefit from the index... so the additional columns in the index ( a.bar and a.foo ) are not beneficial in the sample query posted. From the MySql documentation: MySQL cannot use the index to

Composite Clustered Index in SQL Server

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问题 I have a table with a IDENTITY Column as Primary Key (a classic ID column). SQL Server create automatically a Clustered Index for that Primary Key. My question is: Can I have a only single CLUSTERED INDEX composite with more columns? If yes, how can I drop the default clustered index and recreate a new one with this attributes. Thanks for your support 回答1: Yes, you can only have a single clustered index per table - the data is physically arranged by that index, so you cannot have more than

InnoDB composite indexing order and INSERT performance

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问题 I have a MySQL table with 3 columns on which I'd like to use a multi-column index. Column A is TINYINT, B is SMALLINT and C is VARBINARY (16). Should I use the index as A, B, C, because A has lower granularity than B and B lower than C to achieve maximum INSERT speed? 回答1: (Note: This answer clarifies or disagrees with some of the comments already written.) DELETEs are slowed down because of deleting the index entries. UPDATEs may be slowed down -- it depends on whether an indexed column is

Do mysql composite indexes make some other indexes completely redundant?

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If I have an a composite index on (a, b) I understand that queries only concerned with 'a' will still use the composite index (but not queries concerned with 'b') My question is whether there is any valid reason to have a single-column index on 'a' if I have the (a, b) index? What I've read has seemed vague as to whether the (a,b) index was a complete substitute for a, or merely a "better than nothing" index. This assumes that I do filtering by both a and a,b. I have a table with way too many indexes that is hurting write performance and want to double check before dropping indexes that I'm

Composite Clustered Index in SQL Server

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I have a table with a IDENTITY Column as Primary Key (a classic ID column). SQL Server create automatically a Clustered Index for that Primary Key. My question is: Can I have a only single CLUSTERED INDEX composite with more columns? If yes, how can I drop the default clustered index and recreate a new one with this attributes. Thanks for your support Yes, you can only have a single clustered index per table - the data is physically arranged by that index, so you cannot have more than one. I would however not advise to use a composite clustered index. Why? Because the clustered index should