# Enhancements of Parallel Query

> Source chapter 3.1 for PostgreSQL 11.

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LLMS index: [llms.txt](/en/llms.txt)

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<div class="feature-source-note"><span class="feature-edition-badge">GA</span><span><strong>Source pages:</strong> PDF 11-13</span><span><a href="https://github.com/nori-shinoda/documents/blob/1773200c5362476c4a549222b7d17fa91f546cae/postgresql11_ga_new_features_en_20181022-1.pdf">Original PDF on GitHub</a></span></div>

<p class="feature-page-marker" id="pgfeat-11-parallel-pdf-page-11">PDF page 11</p>

In PostgreSQL 11, parallel queries can now be used in many situations.

#### 3.1.1 Parallel Hash {#pg11-3-1-1-parallel-hash}

Hash joins and hashing can now be processed in parallel. Parallel Hash or Parallel Hash Join is displayed in the execution plan as follows.

**Example 4.** Parallel Hash

```text
postgres=>  EXPLAIN SELECT COUNT(*) FROM hash1 INNER JOIN hash2 ON
hash1.c1 = hash2.c1 ;
                                   QUERY PLAN
-----------------------------------------------------------------------
Finalize Aggregate  (cost=368663.94..368663.95 rows=1 width=8)
   ->  Gather  (cost=368663.73..368663.94 rows=2 width=8)
         Workers Planned: 2
         ->  Partial Aggregate  (cost=367663.73..367663.74 rows=1
width=8)
               ->  Parallel Hash Join  (cost=164082.00..357247.06
rows=4166667 width=0)
                     Hash Cond: (hash2.c1 = hash1.c1)
                     ->  Parallel Seq Scan on hash2  (cost=0.00..95722.40
rows=4166740 width=6)
                     ->  Parallel Hash  (cost=95721.67..95721.67
rows=4166667 width=6)
                           ->  Parallel Seq Scan on hash1
(cost=0.00..95721.67 rows=4166667 width=6)
(9 rows)
```

#### 3.1.2 Parallel Append {#pg11-3-1-2-parallel-append}

Append processing can now be executed in parallel. Parallel Append is displayed in the execution plan.

<p class="feature-page-marker" id="pgfeat-11-parallel-pdf-page-12">PDF page 12</p>

**Example 5.** Parallel Append

```text
postgres=> EXPLAIN SELECT COUNT(*) FROM data1 UNION ALL SELECT COUNT(*) FROM
data2 ;
                                QUERY PLAN
----------------------------------------------------------------------------
Gather  (cost=180053.25..180054.25 rows=2 width=8)
   Workers Planned: 2
   ->  Parallel Append  (cost=179053.25..179054.05 rows=1 width=8)
         ->  Aggregate  (cost=179054.02..179054.04 rows=1 width=8)
               ->  Seq Scan on data1  (cost=0.00..154054.22 rows=9999922 width=0)
         ->  Aggregate  (cost=179053.25..179053.26 rows=1 width=8)
               ->  Seq Scan on data2  (cost=0.00..154053.60 rows=9999860 width=0)
(7 rows)
```

#### 3.1.3 CREATE TABLE AS SELECT statement {#pg11-3-1-3-create-table-as-select-statement}

The SELECT processing in CREATE TABLE AS SELECT statement can now be executed as a parallel query. It was serial processing on PostgreSQL 10.

**Example 6.** Parallelization of CREATE TABLE AS SELECT statement

```text
postgres=> EXPLAIN CREATE TABLE para1 AS SELECT COUNT(*) FROM data1 ;
                                       QUERY PLAN
-----------------------------------------------------------------------
 Finalize Aggregate  (cost=11614.55..11614.56 rows=1 width=8)
   ->  Gather  (cost=11614.33..11614.54 rows=2 width=8)
         Workers Planned: 2
         ->  Partial Aggregate  (cost=10614.33..10614.34 rows=1 width=8)
               ->  Parallel Seq Scan on data1  (cost=0.00..9572.67
rows=416667 width=0)
(5 rows)
```

#### 3.1.4 CREATE MATERIALIZED VIEW statement {#pg11-3-1-4-create-materialized-view-statement}

The SELECT processing of CREATE MATERIALIZED VIEW statement now works as a parallel query. It was serial running on PostgreSQL 10.

<p class="feature-page-marker" id="pgfeat-11-parallel-pdf-page-13">PDF page 13</p>

**Example 7.** Parallelization of CREATE MATERIALIZED VIEW statement

```text
postgres=> EXPLAIN CREATE MATERIALIZED VIEW mv1 AS SELECT COUNT(*) FROM data1 ;
                                       QUERY PLAN
------------------------------------------------------------------------------
 Finalize Aggregate  (cost=11614.55..11614.56 rows=1 width=8)
   ->  Gather  (cost=11614.33..11614.54 rows=2 width=8)
         Workers Planned: 2
         ->  Partial Aggregate  (cost=10614.33..10614.34 rows=1 width=8)
               ->  Parallel Seq Scan on data1  (cost=0.00..9572.67 rows=416667
width=0)
(5 rows)
```

#### 3.1.5 SELECT INTO statement {#pg11-3-1-5-select-into-statement}

SELECT INTO statement now supports parallel queries.

**Example 8.** Parallelization of SELECT INTO statement

```text
postgres=> EXPLAIN SELECT COUNT(*) INTO val FROM data1 ;
                                       QUERY PLAN
--------------------------------------------------------------------------
 Finalize Aggregate  (cost=11614.55..11614.56 rows=1 width=8)
   ->  Gather  (cost=11614.33..11614.54 rows=2 width=8)
         Workers Planned: 2
         ->  Partial Aggregate  (cost=10614.33..10614.34 rows=1 width=8)
               ->  Parallel Seq Scan on data1  (cost=0.00..9572.67
rows=416667 width=0)
(5 rows)
```

#### 3.1.6 CREATE INDEX statement {#pg11-3-1-6-create-index-statement}

BTree index creation processing can now be executed in parallel.