何时以及如何在Grails中创建索引？

Question

In Grails you can add custom indices to your domain classes.

1. Does Grails generate indices by default for my tabels?
2. Is there a rule which columns I have to use for my index?
3. Do my queries change when an index is set?

Answer 1

This isn't really a Grails question, except for the part about when and if Grails creates indexes. You need them like you would in any application that uses a database - create them to improve lookup performance.

Grails doesn't actually create any, Hibernate does that when it generates the DDL that creates your tables. You can see this DDL at any time by running grails schema-export - the generated file will be target/ddl.sql .

In general you'll see unique constraints which will typically create a unique index, and in MySQL and some other databases you'll see indexes created on foreign keys (but this isn't done for Oracle for some reason).

There is some mapping support for getting Hibernate to create indexes as you noted in your question, but in general you'll need to create them yourself since they are often database-specific. Use the http://grails.org/plugin/database-migration plugin for this.

Answer 2

In general you will use indexes on columns that are part of frequent queries and queries with high execution cost. This will happen using any relational database and any development framework.

About Grails, I found this post very useful in how indexes are defined in Grails: http://grails.asia/grails-how-to-create-custom-table-index-or-composite-index