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Project: http://git-wip-us.apache.org/repos/asf/incubator-hawq-docs/repo Commit: http://git-wip-us.apache.org/repos/asf/incubator-hawq-docs/commit/eaf883c7 Tree: http://git-wip-us.apache.org/repos/asf/incubator-hawq-docs/tree/eaf883c7 Diff: http://git-wip-us.apache.org/repos/asf/incubator-hawq-docs/diff/eaf883c7 Branch: refs/heads/develop Commit: eaf883c7fa88dabf6ad8c7a799c2b4cc4cf348b3 Parents: 1014d88 Author: Jane Beckman <[email protected]> Authored: Mon Oct 31 12:04:46 2016 -0700 Committer: Jane Beckman <[email protected]> Committed: Mon Oct 31 12:04:46 2016 -0700 ---------------------------------------------------------------------- bestpractices/querying_data_bestpractices.html.md.erb | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) ---------------------------------------------------------------------- http://git-wip-us.apache.org/repos/asf/incubator-hawq-docs/blob/eaf883c7/bestpractices/querying_data_bestpractices.html.md.erb ---------------------------------------------------------------------- diff --git a/bestpractices/querying_data_bestpractices.html.md.erb b/bestpractices/querying_data_bestpractices.html.md.erb index 08a8e02..ef41fbf 100644 --- a/bestpractices/querying_data_bestpractices.html.md.erb +++ b/bestpractices/querying_data_bestpractices.html.md.erb @@ -16,7 +16,7 @@ If a query performs poorly, examine its query plan and ask the following questio If the plan is not choosing the optimal join order, set `join_collapse_limit=1` and use explicit `JOIN` syntax in your SQL statement to force the legacy query optimizer (planner) to the specified join order. You can also collect more statistics on the relevant join columns. - **Does the optimizer selectively scan partitioned tables?** If you use table partitioning, is the optimizer selectively scanning only the child tables required to satisfy the query predicates? Scans of the parent tables should return 0 rows since the parent tables do not contain any data. See [Verifying Your Partition Strategy](../ddl/ddl-partition.html#topic74) for an example of a query plan that shows a selective partition scan. -- **Does the optimizer choose hash aggregate and hash join operations where applicable?** Hash operations are typically much faster than other types of joins or aggregations. Row comparison and sorting is done in memory rather than reading/writing from disk. To enable the query optimizer to choose hash operations, there must be sufficient memory available to hold the estimated number of rows. You may wish to run an `EXPLAIN ANALYZE` for the query to show which plan operations spilled to disk, how much work memory they used, and how much memory was required to avoid spilling to disk. For example: +- **Does the optimizer choose hash aggregate and hash join operations where applicable?** Hash operations are typically much faster than other types of joins or aggregations. Row comparison and sorting is done in memory rather than reading/writing from disk. To enable the query optimizer to choose hash operations, there must be sufficient memory available to hold the estimated number of rows. Run an `EXPLAIN ANALYZE` for the query to show which plan operations spilled to disk, how much work memory they used, and how much memory was required to avoid spilling to disk. For example: `Work_mem used: 23430K bytes avg, 23430K bytes max (seg0). Work_mem wanted: 33649K bytes avg, 33649K bytes max (seg0) to lessen workfile I/O affecting 2 workers.`
