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new 488336e edit semijoin option description
488336e is described below
commit 488336e7f5c8bdd514c92fd142996d8de7da28c4
Author: Bridget Bevens <[email protected]>
AuthorDate: Thu Dec 20 18:16:23 2018 -0800
edit semijoin option description
---
...t-based-and-hash-based-memory-constrained-operators.md | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)
diff --git
a/_docs/performance-tuning/query-plans-and-tuning/050-sort-based-and-hash-based-memory-constrained-operators.md
b/_docs/performance-tuning/query-plans-and-tuning/050-sort-based-and-hash-based-memory-constrained-operators.md
index 9de96f0..9ab136f 100644
---
a/_docs/performance-tuning/query-plans-and-tuning/050-sort-based-and-hash-based-memory-constrained-operators.md
+++
b/_docs/performance-tuning/query-plans-and-tuning/050-sort-based-and-hash-based-memory-constrained-operators.md
@@ -1,6 +1,6 @@
---
title: "Sort-Based and Hash-Based Memory-Constrained Operators"
-date: 2018-12-19
+date: 2018-12-21
parent: "Query Plans and Tuning"
---
@@ -11,7 +11,7 @@ Drill supports the following memory-intensive operators,
which can temporarily s
- Hash-Aggregate
Drill only uses the External Sort operator to sort data. Drill uses the
Hash-Aggregate operator to aggregate data. Alternatively, Drill can sort the
data and then use the (lightweight) Streaming-Aggregate operator to aggregate
data.
-Drill uses the Hash-Join operator to join data. Drill 1.15 introduces
semi-join functionality inside the Hash-Join operator to improve query
performance. Semi-joins remove the distinct processing below the Hash-Join and
eliminate the overhead incurred from using a Hash Aggregate. Prior to Drill
1.15 (or when [semi-join functionality is
disabled]({{site.baseurl}}/docs/sort-based-and-hash-based-memory-constrained-operators/#disabling-the-hash-operators)),
Drill uses a Distinct Hash Aggrega [...]
+Drill uses the Hash-Join operator to join data. Drill 1.15 introduces
semi-join functionality inside the Hash-Join operator to improve query
performance. Semi-joins remove the distinct processing below the Hash-Join and
eliminate the overhead incurred from using a Hash Aggregate. Prior to Drill
1.15 (or when [semi-join functionality is
disabled]({{site.baseurl}}/docs/sort-based-and-hash-based-memory-constrained-operators/#disabling-the-hash-operators)),
Drill used a Distinct Hash Aggrega [...]
The memory configuration in Drill is specified as the memory limit per-query,
per-node. The allocated memory is equally divided among all instances of the
spillable operators (per query on each node). The number of instances is the
number of spillable operators in the query plan multiplied by the maximal
degree of parallelism. The maximal degree of parallelism is the number of minor
fragments required to perform the work for each instance of a spillable
operator. When an instance of a sp [...]
@@ -104,7 +104,8 @@ Enables or disables hash aggregation; otherwise, Drill does
a sort-based aggrega
Enables or disables hash joins. This option is enabled by default. Drill
assumes that a query will have adequate memory to complete and tries to use the
fastest operations possible. Prior to Drill 1.14, the Hash-Join operator used
an uncontrolled amount of memory (up to 10 GB), after which the operator ran
out of memory. As of Drill 1.14, this operator can spill to disk. This option
is enabled by default.
- **planner.enable_semijoin**
-Enables or disables semi-joins. This option is enabled by default and only
works when the `planner.enable_hashjoin` option is also enabled. When enabled,
Drill uses semi-joins to remove the distinct processing below the Hash Join and
sets the semi-join flag in the Hash Join flag, as shown in the following
example:
+Enables or disables semi-join functionality inside the Hash Join. This option
is enabled by default. When enabled, a semi-join flag inside the HashJoin flag
is set to true, and Drill uses a semi-join to remove the distinct processing
below the Hash Join. When disabled, Drill can still perform semi-joins, but the
semi-joins are performed outside of the Hash Join, as shown in the following
example:
+
###Example: Query Plan with and without Semi-Join
@@ -129,9 +130,11 @@ In the following query plan, you can see the HashAgg
before the HashJoin. In the
**Semi-Join Enabled**
In the following query plan, you can see that the HashAgg is absent. In the
HashJoin flag, you can see that semi-join flag is set to true, indicating that
a semi-join was used. Using the semi-join optimizes the query by reducing the
amount of processing that Drill must perform on data.
- EXPLAIN PLAN FOR SELECT employee_id, full_name FROM cp.`employee.json`
WHERE employee_id IN (SELECT employee_id FROM cp.`employee.json`);
-
--------------------------------------------------------------------------------+
- | text
|
+----------------------------------------------------------------------------------+
+ EXPLAIN PLAN FOR SELECT employee_id, full_name FROM cp.`employee.json`
WHERE employee_id IN (SELECT employee_id FROM cp.`employee.json`);
+
+
+----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
+ | text
|
+
+----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| 00-00 Screen
00-01 Project(employee_id=[$0], full_name=[$1])
00-02 HashJoin(condition=[=($0, $2)], joinType=[inner],
semi-join: =[true])
