Commiteab8195

Etsuro Fujita

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Doc: Further update documentation for asynchronous execution.

Add a note about asynchronous execution by postgres_fdw when applied toAppend nodes that contain synchronous subplan(s) as well. Follow-up forcommit27e1f14.Andrey Lepikhov and Etsuro FujitaDiscussion:https://postgr.es/m/58fa2aa5-07f5-80b5-59a1-fec8a349fee7%40postgrespro.ru

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Original file line number	Diff line number	Diff line change
`@@ -411,6 +411,19 @@ OPTIONS (ADD password_required 'false');`
`411`	`411`	`In such a case, it may be more performant to disable this option to`
`412`	`412`	`eliminate the overhead associated with running queries asynchronously.`
`413`	`413`	`</para>`
	`414`	`+`
	`415`	`+ <para>`
	`416`	`+ Asynchronous execution is applied even when an`
	`417`	`+ <structname>Append</structname> node contains subplan(s) executed`
	`418`	`+ synchronously as well as subplan(s) executed asynchronously.`
	`419`	`+ In such a case, if the asynchronous subplans are ones processed using`
	`420`	`+ <filename>postgres_fdw</filename>, tuples from the asynchronous`
	`421`	`+ subplans are not returned until after at least one synchronous subplan`
	`422`	`+ returns all tuples, as that subplan is executed while the asynchronous`
	`423`	`+ subplans are waiting for the results of asynchronous queries sent to`
	`424`	`+ foreign servers.`
	`425`	`+ This behavior might change in a future release.`
	`426`	`+ </para>`
`414`	`427`	`</listitem>`
`415`	`428`	`</varlistentry>`
`416`	`429`

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