Commit1a219fa

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Add header comments to execParallel.c and nodeGather.c.

Patch by me, per a note from Simon Riggs. Reviewed by Amit Kapilaand Amit Langote.

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`@@ -6,6 +6,14 @@`
`6`	`6`	`* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group`
`7`	`7`	`* Portions Copyright (c) 1994, Regents of the University of California`
`8`	`8`	`*`
	`9`	`+ * This file contains routines that are intended to support setting up,`
	`10`	`+ * using, and tearing down a ParallelContext from within the PostgreSQL`
	`11`	`+ * executor. The ParallelContext machinery will handle starting the`
	`12`	`+ * workers and ensuring that their state generally matches that of the`
	`13`	`+ * leader; see src/backend/access/transam/README.parallel for details.`
	`14`	`+ * However, we must save and restore relevant executor state, such as`
	`15`	`+ * any ParamListInfo associated with the query, buffer usage info, and`
	`16`	`+ * the actual plan to be passed down to the worker.`
`9`	`17`	`*`
`10`	`18`	`* IDENTIFICATION`
`11`	`19`	`* src/backend/executor/execParallel.c`

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`@@ -6,6 +6,20 @@`
`6`	`6`	`* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group`
`7`	`7`	`* Portions Copyright (c) 1994, Regents of the University of California`
`8`	`8`	`*`
	`9`	`+ * A Gather executor launches parallel workers to run multiple copies of a`
	`10`	`+ * plan. It can also run the plan itself, if the workers are not available`
	`11`	`+ * or have not started up yet. It then merges all of the results it produces`
	`12`	`+ * and the results from the workers into a single output stream. Therefore,`
	`13`	`+ * it will normally be used with a plan where running multiple copies of the`
	`14`	`+ * same plan does not produce duplicate output, such as PartialSeqScan.`
	`15`	`+ *`
	`16`	`+ * Alternatively, a Gather node can be configured to use just one worker`
	`17`	`+ * and the single-copy flag can be set. In this case, the Gather node will`
	`18`	`+ * run the plan in one worker and will not execute the plan itself. In`
	`19`	`+ * this case, it simply returns whatever tuples were returned by the worker.`
	`20`	`+ * If a worker cannot be obtained, then it will run the plan itself and`
	`21`	`+ * return the results. Therefore, a plan used with a single-copy Gather`
	`22`	`+ * node need not be parallel-aware.`
`9`	`23`	`*`
`10`	`24`	`* IDENTIFICATION`
`11`	`25`	`* src/backend/executor/nodeGather.c`

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