From 05c8482f7f69a954fd65fce85f896e848fc48197 Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Wed, 10 Mar 2021 07:38:58 +0530 Subject: Enable parallel SELECT for "INSERT INTO ... SELECT ...". Parallel SELECT can't be utilized for INSERT in the following cases: - INSERT statement uses the ON CONFLICT DO UPDATE clause - Target table has a parallel-unsafe: trigger, index expression or predicate, column default expression or check constraint - Target table has a parallel-unsafe domain constraint on any column - Target table is a partitioned table with a parallel-unsafe partition key expression or support function The planner is updated to perform additional parallel-safety checks for the cases listed above, for determining whether it is safe to run INSERT in parallel-mode with an underlying parallel SELECT. The planner will consider using parallel SELECT for "INSERT INTO ... SELECT ...", provided nothing unsafe is found from the additional parallel-safety checks, or from the existing parallel-safety checks for SELECT. While checking parallel-safety, we need to check it for all the partitions on the table which can be costly especially when we decide not to use a parallel plan. So, in a separate patch, we will introduce a GUC and or a reloption to enable/disable parallelism for Insert statements. Prior to entering parallel-mode for the execution of INSERT with parallel SELECT, a TransactionId is acquired and assigned to the current transaction state. This is necessary to prevent the INSERT from attempting to assign the TransactionId whilst in parallel-mode, which is not allowed. This approach has a disadvantage in that if the underlying SELECT does not return any rows, then the TransactionId is not used, however that shouldn't happen in practice in many cases. Author: Greg Nancarrow, Amit Langote, Amit Kapila Reviewed-by: Amit Langote, Hou Zhijie, Takayuki Tsunakawa, Antonin Houska, Bharath Rupireddy, Dilip Kumar, Vignesh C, Zhihong Yu, Amit Kapila Tested-by: Tang, Haiying Discussion: https://postgr.es/m/CAJcOf-cXnB5cnMKqWEp2E2z7Mvcd04iLVmV=qpFJrR3AcrTS3g@mail.gmail.com Discussion: https://postgr.es/m/CAJcOf-fAdj=nDKMsRhQzndm-O13NY4dL6xGcEvdX5Xvbbi0V7g@mail.gmail.com --- doc/src/sgml/parallel.sgml | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'doc/src') diff --git a/doc/src/sgml/parallel.sgml b/doc/src/sgml/parallel.sgml index c81abff48d3..cec1329e259 100644 --- a/doc/src/sgml/parallel.sgml +++ b/doc/src/sgml/parallel.sgml @@ -146,7 +146,9 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%'; a CTE, no parallel plans for that query will be generated. As an exception, the commands CREATE TABLE ... AS, SELECT INTO, and CREATE MATERIALIZED VIEW which create a new - table and populate it can use a parallel plan. + table and populate it can use a parallel plan. Another exception is the command + INSERT INTO ... SELECT ... which can use a parallel plan for + the underlying SELECT part of the query. -- cgit v1.2.3