/*-------------------------------------------------------------------------
 *
 * nodeMergeAppend.c
 *	  routines to handle MergeAppend nodes.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/executor/nodeMergeAppend.c
 *
 *-------------------------------------------------------------------------
 */
/* INTERFACE ROUTINES
 *		ExecInitMergeAppend		- initialize the MergeAppend node
 *		ExecMergeAppend			- retrieve the next tuple from the node
 *		ExecEndMergeAppend		- shut down the MergeAppend node
 *		ExecReScanMergeAppend	- rescan the MergeAppend node
 *
 *	 NOTES
 *		A MergeAppend node contains a list of one or more subplans.
 *		These are each expected to deliver tuples that are sorted according
 *		to a common sort key.  The MergeAppend node merges these streams
 *		to produce output sorted the same way.
 *
 *		MergeAppend nodes don't make use of their left and right
 *		subtrees, rather they maintain a list of subplans so
 *		a typical MergeAppend node looks like this in the plan tree:
 *
 *				   ...
 *				   /
 *				MergeAppend---+------+------+--- nil
 *				/	\		  |		 |		|
 *			  nil	nil		 ...    ...    ...
 *								 subplans
 */

#include "postgres.h"

#include "executor/execdebug.h"
#include "executor/nodeMergeAppend.h"

#include "lib/binaryheap.h"

/*
 * We have one slot for each item in the heap array.  We use SlotNumber
 * to store slot indexes.  This doesn't actually provide any formal
 * type-safety, but it makes the code more self-documenting.
 */
typedef int32 SlotNumber;

static int	heap_compare_slots(Datum a, Datum b, void *arg);


/* ----------------------------------------------------------------
 *		ExecInitMergeAppend
 *
 *		Begin all of the subscans of the MergeAppend node.
 * ----------------------------------------------------------------
 */
MergeAppendState *
ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
{
	MergeAppendState *mergestate = makeNode(MergeAppendState);
	PlanState **mergeplanstates;
	int			nplans;
	int			i;
	ListCell   *lc;

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

	/*
	 * Set up empty vector of subplan states
	 */
	nplans = list_length(node->mergeplans);

	mergeplanstates = (PlanState **) palloc0(nplans * sizeof(PlanState *));

	/*
	 * create new MergeAppendState for our node
	 */
	mergestate->ps.plan = (Plan *) node;
	mergestate->ps.state = estate;
	mergestate->mergeplans = mergeplanstates;
	mergestate->ms_nplans = nplans;

	mergestate->ms_slots = (TupleTableSlot **) palloc0(sizeof(TupleTableSlot *) * nplans);
	mergestate->ms_heap = binaryheap_allocate(nplans, heap_compare_slots,
											  mergestate);

	/*
	 * Miscellaneous initialization
	 *
	 * MergeAppend plans don't have expression contexts because they never
	 * call ExecQual or ExecProject.
	 */

	/*
	 * MergeAppend nodes do have Result slots, which hold pointers to tuples,
	 * so we have to initialize them.
	 */
	ExecInitResultTupleSlot(estate, &mergestate->ps);

	/*
	 * call ExecInitNode on each of the plans to be executed and save the
	 * results into the array "mergeplans".
	 */
	i = 0;
	foreach(lc, node->mergeplans)
	{
		Plan	   *initNode = (Plan *) lfirst(lc);

		mergeplanstates[i] = ExecInitNode(initNode, estate, eflags);
		i++;
	}

	/*
	 * initialize output tuple type
	 */
	ExecAssignResultTypeFromTL(&mergestate->ps);
	mergestate->ps.ps_ProjInfo = NULL;

	/*
	 * initialize sort-key information
	 */
	mergestate->ms_nkeys = node->numCols;
	mergestate->ms_sortkeys = palloc0(sizeof(SortSupportData) * node->numCols);

	for (i = 0; i < node->numCols; i++)
	{
		SortSupport sortKey = mergestate->ms_sortkeys + i;

		sortKey->ssup_cxt = CurrentMemoryContext;
		sortKey->ssup_collation = node->collations[i];
		sortKey->ssup_nulls_first = node->nullsFirst[i];
		sortKey->ssup_attno = node->sortColIdx[i];

		/*
		 * It isn't feasible to perform abbreviated key conversion, since
		 * tuples are pulled into mergestate's binary heap as needed.  It
		 * would likely be counter-productive to convert tuples into an
		 * abbreviated representation as they're pulled up, so opt out of that
		 * additional optimization entirely.
		 */
		sortKey->abbreviate = false;

		PrepareSortSupportFromOrderingOp(node->sortOperators[i], sortKey);
	}

	/*
	 * initialize to show we have not run the subplans yet
	 */
	mergestate->ms_initialized = false;

	return mergestate;
}

/* ----------------------------------------------------------------
 *	   ExecMergeAppend
 *
 *		Handles iteration over multiple subplans.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecMergeAppend(MergeAppendState *node)
{
	TupleTableSlot *result;
	SlotNumber	i;

	if (!node->ms_initialized)
	{
		/*
		 * First time through: pull the first tuple from each subplan, and set
		 * up the heap.
		 */
		for (i = 0; i < node->ms_nplans; i++)
		{
			node->ms_slots[i] = ExecProcNode(node->mergeplans[i]);
			if (!TupIsNull(node->ms_slots[i]))
				binaryheap_add_unordered(node->ms_heap, Int32GetDatum(i));
		}
		binaryheap_build(node->ms_heap);
		node->ms_initialized = true;
	}
	else
	{
		/*
		 * Otherwise, pull the next tuple from whichever subplan we returned
		 * from last time, and reinsert the subplan index into the heap,
		 * because it might now compare differently against the existing
		 * elements of the heap.  (We could perhaps simplify the logic a bit
		 * by doing this before returning from the prior call, but it's better
		 * to not pull tuples until necessary.)
		 */
		i = DatumGetInt32(binaryheap_first(node->ms_heap));
		node->ms_slots[i] = ExecProcNode(node->mergeplans[i]);
		if (!TupIsNull(node->ms_slots[i]))
			binaryheap_replace_first(node->ms_heap, Int32GetDatum(i));
		else
			(void) binaryheap_remove_first(node->ms_heap);
	}

	if (binaryheap_empty(node->ms_heap))
	{
		/* All the subplans are exhausted, and so is the heap */
		result = ExecClearTuple(node->ps.ps_ResultTupleSlot);
	}
	else
	{
		i = DatumGetInt32(binaryheap_first(node->ms_heap));
		result = node->ms_slots[i];
	}

	return result;
}

/*
 * Compare the tuples in the two given slots.
 */
static int32
heap_compare_slots(Datum a, Datum b, void *arg)
{
	MergeAppendState *node = (MergeAppendState *) arg;
	SlotNumber	slot1 = DatumGetInt32(a);
	SlotNumber	slot2 = DatumGetInt32(b);

	TupleTableSlot *s1 = node->ms_slots[slot1];
	TupleTableSlot *s2 = node->ms_slots[slot2];
	int			nkey;

	Assert(!TupIsNull(s1));
	Assert(!TupIsNull(s2));

	for (nkey = 0; nkey < node->ms_nkeys; nkey++)
	{
		SortSupport sortKey = node->ms_sortkeys + nkey;
		AttrNumber	attno = sortKey->ssup_attno;
		Datum		datum1,
					datum2;
		bool		isNull1,
					isNull2;
		int			compare;

		datum1 = slot_getattr(s1, attno, &isNull1);
		datum2 = slot_getattr(s2, attno, &isNull2);

		compare = ApplySortComparator(datum1, isNull1,
									  datum2, isNull2,
									  sortKey);
		if (compare != 0)
			return -compare;
	}
	return 0;
}

/* ----------------------------------------------------------------
 *		ExecEndMergeAppend
 *
 *		Shuts down the subscans of the MergeAppend node.
 *
 *		Returns nothing of interest.
 * ----------------------------------------------------------------
 */
void
ExecEndMergeAppend(MergeAppendState *node)
{
	PlanState **mergeplans;
	int			nplans;
	int			i;

	/*
	 * get information from the node
	 */
	mergeplans = node->mergeplans;
	nplans = node->ms_nplans;

	/*
	 * shut down each of the subscans
	 */
	for (i = 0; i < nplans; i++)
		ExecEndNode(mergeplans[i]);
}

void
ExecReScanMergeAppend(MergeAppendState *node)
{
	int			i;

	for (i = 0; i < node->ms_nplans; i++)
	{
		PlanState  *subnode = node->mergeplans[i];

		/*
		 * ExecReScan doesn't know about my subplans, so I have to do
		 * changed-parameter signaling myself.
		 */
		if (node->ps.chgParam != NULL)
			UpdateChangedParamSet(subnode, node->ps.chgParam);

		/*
		 * If chgParam of subnode is not null then plan will be re-scanned by
		 * first ExecProcNode.
		 */
		if (subnode->chgParam == NULL)
			ExecReScan(subnode);
	}
	binaryheap_reset(node->ms_heap);
	node->ms_initialized = false;
}
