What is a use case for a join index?
JOIN INDEX is a materialized view. Its definition is permanently stored and the data is updated whenever the base tables referred in the join index is updated. JOIN INDEX may contain one or more tables and also contain pre-aggregated data. Join indexes are mainly used for improving the performance.
What are indexes in SQL?
An index is an on-disk structure associated with a table or view that speeds retrieval of rows from the table or view. These keys are stored in a structure (B-tree) that enables SQL Server to find the row or rows associated with the key values quickly and efficiently.
When to use an index in a query?
There are many different scenarios when an index can help the performance of a query and ensuring that the columns that make up your JOIN predicate is an important one. In order to illustrate this point let’s take a look at a simple query that joins the Parent and Child tables.
Why does MySQL not use Y index for join?
It uses a full index scan of last_updated on the first table for sorting, but does not use an y index for join ( type: index in explain). This is very bad for performance and kills the whole database server, since this is a very frequent query.
What happens when you add index to join columns?
Let’s confirm our assumption by taking a look at the SQL Profiler output of both queries. We see below that we were correct in our assumption. With the index added the query ran much faster, used much less CPU and performed way fewer reads. There is one other thing I’d like to mention when it comes to adding indexes on join columns.
Do you set index on join clauses or where clauses?
Do you set an index on JOIN clauses or where clauses, or both? SELECT t1, t2 FROM t1 LEFT JOIN t2 ON (t1.id = t2.id AND t2.userid = @userid) WHERE t1.enabled = 1 AND t1.startDate <= ??? AND (t1.counter = -1 OR t2.counter IS NULL OR (t1.counter > t2.counter)