/*-------------------------------------------------------------------------
 *
 * nodeForeignscan.c
 *	  Routines to support scans of foreign tables
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/executor/nodeForeignscan.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *
 *		ExecForeignScan			scans a foreign table.
 *		ExecInitForeignScan		creates and initializes state info.
 *		ExecReScanForeignScan	rescans the foreign relation.
 *		ExecEndForeignScan		releases any resources allocated.
 */
#include "postgres.h"

#include "executor/executor.h"
#include "executor/nodeForeignscan.h"
#include "foreign/fdwapi.h"
#include "utils/memutils.h"
#include "utils/rel.h"

static TupleTableSlot *ForeignNext(ForeignScanState *node);
static bool ForeignRecheck(ForeignScanState *node, TupleTableSlot *slot);


/* ----------------------------------------------------------------
 *		ForeignNext
 *
 *		This is a workhorse for ExecForeignScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ForeignNext(ForeignScanState *node)
{
	TupleTableSlot *slot;
	ForeignScan *plan = (ForeignScan *) node->ss.ps.plan;
	ExprContext *econtext = node->ss.ps.ps_ExprContext;
	MemoryContext oldcontext;

	/* Call the Iterate function in short-lived context */
	oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
	slot = node->fdwroutine->IterateForeignScan(node);
	MemoryContextSwitchTo(oldcontext);

	/*
	 * If any system columns are requested, we have to force the tuple into
	 * physical-tuple form to avoid "cannot extract system attribute from
	 * virtual tuple" errors later.  We also insert a valid value for
	 * tableoid, which is the only actually-useful system column.
	 */
	if (plan->fsSystemCol && !TupIsNull(slot))
	{
		HeapTuple	tup = ExecMaterializeSlot(slot);

		tup->t_tableOid = RelationGetRelid(node->ss.ss_currentRelation);
	}

	return slot;
}

/*
 * ForeignRecheck -- access method routine to recheck a tuple in EvalPlanQual
 */
static bool
ForeignRecheck(ForeignScanState *node, TupleTableSlot *slot)
{
	FdwRoutine *fdwroutine = node->fdwroutine;
	ExprContext *econtext;

	/*
	 * extract necessary information from foreign scan node
	 */
	econtext = node->ss.ps.ps_ExprContext;

	/* Does the tuple meet the remote qual condition? */
	econtext->ecxt_scantuple = slot;

	ResetExprContext(econtext);

	/*
	 * If an outer join is pushed down, RecheckForeignScan may need to store a
	 * different tuple in the slot, because a different set of columns may go
	 * to NULL upon recheck.  Otherwise, it shouldn't need to change the slot
	 * contents, just return true or false to indicate whether the quals still
	 * pass.  For simple cases, setting fdw_recheck_quals may be easier than
	 * providing this callback.
	 */
	if (fdwroutine->RecheckForeignScan &&
		!fdwroutine->RecheckForeignScan(node, slot))
		return false;

	return ExecQual(node->fdw_recheck_quals, econtext, false);
}

/* ----------------------------------------------------------------
 *		ExecForeignScan(node)
 *
 *		Fetches the next tuple from the FDW, checks local quals, and
 *		returns it.
 *		We call the ExecScan() routine and pass it the appropriate
 *		access method functions.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecForeignScan(ForeignScanState *node)
{
	return ExecScan((ScanState *) node,
					(ExecScanAccessMtd) ForeignNext,
					(ExecScanRecheckMtd) ForeignRecheck);
}


/* ----------------------------------------------------------------
 *		ExecInitForeignScan
 * ----------------------------------------------------------------
 */
ForeignScanState *
ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
{
	ForeignScanState *scanstate;
	Relation	currentRelation = NULL;
	Index		scanrelid = node->scan.scanrelid;
	Index		tlistvarno;
	FdwRoutine *fdwroutine;

	/* check for unsupported flags */
	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));

	/*
	 * create state structure
	 */
	scanstate = makeNode(ForeignScanState);
	scanstate->ss.ps.plan = (Plan *) node;
	scanstate->ss.ps.state = estate;

	/*
	 * Miscellaneous initialization
	 *
	 * create expression context for node
	 */
	ExecAssignExprContext(estate, &scanstate->ss.ps);

	scanstate->ss.ps.ps_TupFromTlist = false;

	/*
	 * initialize child expressions
	 */
	scanstate->ss.ps.targetlist = (List *)
		ExecInitExpr((Expr *) node->scan.plan.targetlist,
					 (PlanState *) scanstate);
	scanstate->ss.ps.qual = (List *)
		ExecInitExpr((Expr *) node->scan.plan.qual,
					 (PlanState *) scanstate);
	scanstate->fdw_recheck_quals = (List *)
		ExecInitExpr((Expr *) node->fdw_recheck_quals,
					 (PlanState *) scanstate);

	/*
	 * tuple table initialization
	 */
	ExecInitResultTupleSlot(estate, &scanstate->ss.ps);
	ExecInitScanTupleSlot(estate, &scanstate->ss);

	/*
	 * open the base relation, if any, and acquire an appropriate lock on it;
	 * also acquire function pointers from the FDW's handler
	 */
	if (scanrelid > 0)
	{
		currentRelation = ExecOpenScanRelation(estate, scanrelid, eflags);
		scanstate->ss.ss_currentRelation = currentRelation;
		fdwroutine = GetFdwRoutineForRelation(currentRelation, true);
	}
	else
	{
		/* We can't use the relcache, so get fdwroutine the hard way */
		fdwroutine = GetFdwRoutineByServerId(node->fs_server);
	}

	/*
	 * Determine the scan tuple type.  If the FDW provided a targetlist
	 * describing the scan tuples, use that; else use base relation's rowtype.
	 */
	if (node->fdw_scan_tlist != NIL || currentRelation == NULL)
	{
		TupleDesc	scan_tupdesc;

		scan_tupdesc = ExecTypeFromTL(node->fdw_scan_tlist, false);
		ExecAssignScanType(&scanstate->ss, scan_tupdesc);
		/* Node's targetlist will contain Vars with varno = INDEX_VAR */
		tlistvarno = INDEX_VAR;
	}
	else
	{
		ExecAssignScanType(&scanstate->ss, RelationGetDescr(currentRelation));
		/* Node's targetlist will contain Vars with varno = scanrelid */
		tlistvarno = scanrelid;
	}

	/*
	 * Initialize result tuple type and projection info.
	 */
	ExecAssignResultTypeFromTL(&scanstate->ss.ps);
	ExecAssignScanProjectionInfoWithVarno(&scanstate->ss, tlistvarno);

	/*
	 * Initialize FDW-related state.
	 */
	scanstate->fdwroutine = fdwroutine;
	scanstate->fdw_state = NULL;

	/* Initialize any outer plan. */
	if (outerPlan(node))
		outerPlanState(scanstate) =
			ExecInitNode(outerPlan(node), estate, eflags);

	/*
	 * Tell the FDW to initialize the scan.
	 */
	fdwroutine->BeginForeignScan(scanstate, eflags);

	return scanstate;
}

/* ----------------------------------------------------------------
 *		ExecEndForeignScan
 *
 *		frees any storage allocated through C routines.
 * ----------------------------------------------------------------
 */
void
ExecEndForeignScan(ForeignScanState *node)
{
	/* Let the FDW shut down */
	node->fdwroutine->EndForeignScan(node);

	/* Shut down any outer plan. */
	if (outerPlanState(node))
		ExecEndNode(outerPlanState(node));

	/* Free the exprcontext */
	ExecFreeExprContext(&node->ss.ps);

	/* clean out the tuple table */
	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
	ExecClearTuple(node->ss.ss_ScanTupleSlot);

	/* close the relation. */
	if (node->ss.ss_currentRelation)
		ExecCloseScanRelation(node->ss.ss_currentRelation);
}

/* ----------------------------------------------------------------
 *		ExecReScanForeignScan
 *
 *		Rescans the relation.
 * ----------------------------------------------------------------
 */
void
ExecReScanForeignScan(ForeignScanState *node)
{
	PlanState  *outerPlan = outerPlanState(node);

	node->fdwroutine->ReScanForeignScan(node);

	/*
	 * If chgParam of subnode is not null then plan will be re-scanned by
	 * first ExecProcNode.  outerPlan may also be NULL, in which case there
	 * is nothing to rescan at all.
	 */
	if (outerPlan != NULL && outerPlan->chgParam == NULL)
		ExecReScan(outerPlan);

	ExecScanReScan(&node->ss);
}
