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mirror of https://github.com/postgres/postgres.git synced 2025-08-25 20:23:07 +03:00

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
This commit is contained in:
Amit Kapila
2021-03-10 07:38:58 +05:30
parent 0ba71107ef
commit 05c8482f7f
17 changed files with 1531 additions and 21 deletions

View File

@@ -1014,6 +1014,32 @@ IsInParallelMode(void)
return CurrentTransactionState->parallelModeLevel != 0;
}
/*
* PrepareParallelModePlanExec
*
* Prepare for entering parallel mode plan execution, based on command-type.
*/
void
PrepareParallelModePlanExec(CmdType commandType)
{
if (IsModifySupportedInParallelMode(commandType))
{
Assert(!IsInParallelMode());
/*
* Prepare for entering parallel mode by assigning a TransactionId.
* Failure to do this now would result in heap_insert() subsequently
* attempting to assign a 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.
*/
(void) GetCurrentTransactionId();
}
}
/*
* CommandCounterIncrement
*/