4/25/2023 0 Comments Redshift temp tableRedistribution of a table for a join only happens temporarily for the life of the join. This has nothing to do with the sortkey column, but everything to do with the distkey of both tables. When two tables being joined do not have matching rows co-located on each slice the system will redistribute one or both of the tables by the join column. Therefore, two rows being joined must physically reside on the same slice. Table Joins: Why is the Example Bad? What Must Redshift Do?Įach Slice has their own memory. Since each slice has their own memory all joining rows must be on the same slice, or Redshift will have to move data around for the join to happen. This means that the matching rows being joined are already on the same slice. This example is the best choice for joining because both tables join on the column DeptNo and DeptNo is the DISTKEY for both tables. Give a specific answer for which page you have chosen.īest Choice Because Two Rows Joining Must Physically be on the Same Slice Compare this page with the previous page and choose which page has the best performance when joining the Emp_Tbl with the Dept_Tbl. Look how the data has been spread across the slices for both tables. Table Joins: Which Join is More Efficient and Why? This Page or Previous Page?Ĭheck out the Data Definition Language (DDL), which means the table CREATE statement for both tables. Give a specific answer for which page you have chosen. Compare this page with the next page and choose which page has the best performance when joining the Emp_Tbl with the Dept_Tbl. Large tables will take a different approach to small tables.Ĭheck out the Data Definition Language (DDL), which means the table CREATE statement for both tables. The number of rows in a table will be a key factor in how you tune tables for joins. Now, let’s do some exercises to explore table joins, and which of the following joins are more efficient (and why). I believe in my 25 years of teaching big data that what you are about to learn might be the most monumental information to upgrade your knowledge. This section will show you how to do just that. But make no mistake about it, you should concentrate your tuning efforts on join efficiency. When you use the create table command to create a permanent table or even a temp table you control the table’s distkey and sortkey, and each column’s data type. Amazon Web Services has made tuning your Redshift Cluster easy. This post is the second part of a three-part series on an Amazon Redshift Data Warehouse.
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