static
inline
double
compute_metric(PGFunction inMetricFn, MemoryContext inMemContext, Datum inVec1,
    Datum inVec2) {
    
    float8          distance;
    MemoryContext   oldContext;
    
    oldContext = MemoryContextSwitchTo(inMemContext);
    
    distance = DatumGetFloat8(DirectFunctionCall2(inMetricFn, inVec1, inVec2));
    
#ifdef GP_VERSION_NUM
    /*
     * Once the direct function calls have leaked enough memory, let's do some
     * garbage collection...
     * The 50k bound here is arbitrary, and motivated by ResetExprContext()
     * in execUtils.c
     */
    if(inMemContext->allBytesAlloc - inMemContext->allBytesFreed > 50000)
        MemoryContextReset(inMemContext);
#else
    /* PostgreSQL does not have the allBytesAlloc and allBytesFreed fields */
    MemoryContextReset(inMemContext);
#endif
    
    MemoryContextSwitchTo(oldContext);    
    return distance;
}

/* Callback to process one heap tuple during IndexBuildHeapScan */
static void
spgistBuildCallback(Relation index, HeapTuple htup, Datum *values,
					bool *isnull, bool tupleIsAlive, void *state)
{
	SpGistBuildState *buildstate = (SpGistBuildState *) state;
	MemoryContext oldCtx;

	/* Work in temp context, and reset it after each tuple */
	oldCtx = MemoryContextSwitchTo(buildstate->tmpCtx);

	/*
	 * Even though no concurrent insertions can be happening, we still might
	 * get a buffer-locking failure due to bgwriter or checkpointer taking a
	 * lock on some buffer.  So we need to be willing to retry.  We can flush
	 * any temp data when retrying.
	 */
	while (!spgdoinsert(index, &buildstate->spgstate, &htup->t_self,
						*values, *isnull))
	{
		MemoryContextReset(buildstate->tmpCtx);
	}

	MemoryContextSwitchTo(oldCtx);
	MemoryContextReset(buildstate->tmpCtx);
}

/*
 * Make union of keys on page
 */
static IndexTuple
PageMakeUnionKey(GistVacuum *gv, Buffer buffer)
{
	Page		page = BufferGetPage(buffer);
	IndexTuple *vec,
				tmp,
				res;
	int			veclen = 0;
	MemoryContext oldCtx = MemoryContextSwitchTo(gv->opCtx);

	vec = gistextractpage(page, &veclen);

	/*
	 * we call gistunion() in temprorary context because user-defined
	 * functions called in gistunion() may do not free all memory
	 */
	tmp = gistunion(gv->index, vec, veclen, &(gv->giststate));
	MemoryContextSwitchTo(oldCtx);

	res = (IndexTuple) palloc(IndexTupleSize(tmp));
	memcpy(res, tmp, IndexTupleSize(tmp));

	ItemPointerSetBlockNumber(&(res->t_tid), BufferGetBlockNumber(buffer));
	GistTupleSetValid(res);

	MemoryContextReset(gv->opCtx);

	return res;
}

static void
elements_array_element_end(void *state, bool isnull)
{
	ElementsState _state = (ElementsState) state;
	MemoryContext old_cxt;
	int			len;
	text	   *val;
	HeapTuple	tuple;
	Datum		values[1];
	static bool nulls[1] = {false};

	/* skip over nested objects */
	if (_state->lex->lex_level != 1)
		return;

	/* use the tmp context so we can clean up after each tuple is done */
	old_cxt = MemoryContextSwitchTo(_state->tmp_cxt);

	len = _state->lex->prev_token_terminator - _state->result_start;
	val = cstring_to_text_with_len(_state->result_start, len);

	values[0] = PointerGetDatum(val);

	tuple = heap_form_tuple(_state->ret_tdesc, values, nulls);

	tuplestore_puttuple(_state->tuple_store, tuple);

	/* clean up and switch back */
	MemoryContextSwitchTo(old_cxt);
	MemoryContextReset(_state->tmp_cxt);
}

/*
 * Reset a BrinMemTuple to initial state.  We return the same tuple, for
 * notational convenience.
 */
BrinMemTuple *
brin_memtuple_initialize(BrinMemTuple *dtuple, BrinDesc *brdesc)
{
	int			i;
	char	   *currdatum;

	MemoryContextReset(dtuple->bt_context);

	currdatum = (char *) dtuple +
		MAXALIGN(sizeof(BrinMemTuple) +
				 sizeof(BrinValues) * brdesc->bd_tupdesc->natts);
	for (i = 0; i < brdesc->bd_tupdesc->natts; i++)
	{
		dtuple->bt_columns[i].bv_allnulls = true;
		dtuple->bt_columns[i].bv_hasnulls = false;

		dtuple->bt_columns[i].bv_attno = i + 1;
		dtuple->bt_columns[i].bv_allnulls = true;
		dtuple->bt_columns[i].bv_hasnulls = false;
		dtuple->bt_columns[i].bv_values = (Datum *) currdatum;
		currdatum += sizeof(Datum) * brdesc->bd_info[i]->oi_nstored;
	}

	return dtuple;
}

/*
 *	gistinsert -- wrapper for GiST tuple insertion.
 *
 *	  This is the public interface routine for tuple insertion in GiSTs.
 *	  It doesn't do any work; just locks the relation and passes the buck.
 */
bool
gistinsert(Relation r, Datum *values, bool *isnull,
		   ItemPointer ht_ctid, Relation heapRel,
		   IndexUniqueCheck checkUnique,
		   IndexInfo *indexInfo)
{
	GISTSTATE  *giststate = (GISTSTATE *) indexInfo->ii_AmCache;
	IndexTuple	itup;
	MemoryContext oldCxt;

	/* Initialize GISTSTATE cache if first call in this statement */
	if (giststate == NULL)
	{
		oldCxt = MemoryContextSwitchTo(indexInfo->ii_Context);
		giststate = initGISTstate(r);
		giststate->tempCxt = createTempGistContext();
		indexInfo->ii_AmCache = (void *) giststate;
		MemoryContextSwitchTo(oldCxt);
	}

	oldCxt = MemoryContextSwitchTo(giststate->tempCxt);

	itup = gistFormTuple(giststate, r,
						 values, isnull, true /* size is currently bogus */ );
	itup->t_tid = *ht_ctid;

	gistdoinsert(r, itup, 0, giststate);

	/* cleanup */
	MemoryContextSwitchTo(oldCxt);
	MemoryContextReset(giststate->tempCxt);

	return false;
}

/*
 * Perform input conversion, given correctly-set-up state information.
 *
 * This is the outer-level entry point for any input conversion.  Internally,
 * the conversion functions recurse directly to each other.
 */
PyObject *
PLy_input_convert(PLyDatumToOb *arg, Datum val)
{
	PyObject   *result;
	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
	MemoryContext scratch_context = PLy_get_scratch_context(exec_ctx);
	MemoryContext oldcontext;

	/*
	 * Do the work in the scratch context to avoid leaking memory from the
	 * datatype output function calls.  (The individual PLyDatumToObFunc
	 * functions can't reset the scratch context, because they recurse and an
	 * inner one might clobber data an outer one still needs.  So we do it
	 * once at the outermost recursion level.)
	 *
	 * We reset the scratch context before, not after, each conversion cycle.
	 * This way we aren't on the hook to release a Python refcount on the
	 * result object in case MemoryContextReset throws an error.
	 */
	MemoryContextReset(scratch_context);

	oldcontext = MemoryContextSwitchTo(scratch_context);

	result = arg->func(arg, val);

	MemoryContextSwitchTo(oldcontext);

	return result;
}

void
gist_redo(XLogReaderState *record)
{
	uint8		info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
	MemoryContext oldCxt;

	/*
	 * GiST indexes do not require any conflict processing. NB: If we ever
	 * implement a similar optimization we have in b-tree, and remove killed
	 * tuples outside VACUUM, we'll need to handle that here.
	 */

	oldCxt = MemoryContextSwitchTo(opCtx);
	switch (info)
	{
		case XLOG_GIST_PAGE_UPDATE:
			gistRedoPageUpdateRecord(record);
			break;
		case XLOG_GIST_PAGE_SPLIT:
			gistRedoPageSplitRecord(record);
			break;
		case XLOG_GIST_CREATE_INDEX:
			gistRedoCreateIndex(record);
			break;
		default:
			elog(PANIC, "gist_redo: unknown op code %u", info);
	}

	MemoryContextSwitchTo(oldCxt);
	MemoryContextReset(opCtx);
}

static void
ginBuildCallback(Relation index, HeapTuple htup, Datum *values,
				 bool *isnull, bool tupleIsAlive, void *state)
{
	GinBuildState *buildstate = (GinBuildState *) state;
	MemoryContext oldCtx;

	if (*isnull)
		return;

	oldCtx = MemoryContextSwitchTo(buildstate->tmpCtx);

	buildstate->indtuples += ginHeapTupleBulkInsert(buildstate, *values, &htup->t_self);

	/* If we've maxed out our available memory, dump everything to the index */
	/* Also dump if the tree seems to be getting too unbalanced */
	if (buildstate->accum.allocatedMemory >= maintenance_work_mem * 1024L ||
		buildstate->accum.maxdepth > GIN_MAX_TREE_DEPTH)
	{
		ItemPointerData *list;
		Datum		entry;
		uint32		nlist;

		while ((list = ginGetEntry(&buildstate->accum, &entry, &nlist)) != NULL)
			ginEntryInsert(index, &buildstate->ginstate, entry, list, nlist, TRUE);

		MemoryContextReset(buildstate->tmpCtx);
		ginInitBA(&buildstate->accum);
	}

	MemoryContextSwitchTo(oldCtx);
}

/*
 * DynamicScan_InitExpr
 * 		Initialize ExprState for a new partition from the plan's expressions
 */
static void
DynamicScan_InitExpr(ScanState* scanState)
{
	MemoryContext oldCxt = NULL;
	MemoryContext partCxt = DynamicScan_GetPartitionMemoryContext(scanState);
	if (NULL != partCxt)
	{
		MemoryContextReset(partCxt);
		/*
		 * Switch to partition memory context to prevent memory leak for
		 * per-partition data structures.
		 */
		oldCxt = MemoryContextSwitchTo(partCxt);
	}

	/*
	 * We might have reset the memory context. Set these dangling
	 * pointers to NULL so that we don't try to pfree them later
	 */
	scanState->ps.ps_ProjInfo = NULL;
	scanState->ps.qual = NULL;
	scanState->ps.targetlist = NULL;
	/* Initialize child expressions */
	scanState->ps.qual = (List*) ExecInitExpr((Expr*) scanState->ps.plan->qual,
			(PlanState*) scanState);
	scanState->ps.targetlist = (List*) ExecInitExpr(
			(Expr*) scanState->ps.plan->targetlist, (PlanState*) scanState);
	ExecAssignScanProjectionInfo(scanState);

	if (NULL != oldCxt)
	{
		MemoryContextSwitchTo(oldCxt);
	}
}

/*
 * execTuplesMatch
 *		Return true if two tuples match in all the indicated fields.
 *
 * This actually implements SQL's notion of "not distinct".  Two nulls
 * match, a null and a not-null don't match.
 *
 * slot1, slot2: the tuples to compare (must have same columns!)
 * numCols: the number of attributes to be examined
 * matchColIdx: array of attribute column numbers
 * eqFunctions: array of fmgr lookup info for the equality functions to use
 * evalContext: short-term memory context for executing the functions
 *
 * NB: evalContext is reset each time!
 */
bool
execTuplesMatch(TupleTableSlot *slot1,
				TupleTableSlot *slot2,
				int numCols,
				AttrNumber *matchColIdx,
				FmgrInfo *eqfunctions,
				MemoryContext evalContext)
{
	MemoryContext oldContext;
	bool		result;
	int			i;

	/* Reset and switch into the temp context. */
	MemoryContextReset(evalContext);
	oldContext = MemoryContextSwitchTo(evalContext);

	/*
	 * We cannot report a match without checking all the fields, but we can
	 * report a non-match as soon as we find unequal fields.  So, start
	 * comparing at the last field (least significant sort key). That's the
	 * most likely to be different if we are dealing with sorted input.
	 */
	result = true;

	for (i = numCols; --i >= 0;)
	{
		AttrNumber	att = matchColIdx[i];
		Datum		attr1,
					attr2;
		bool		isNull1,
					isNull2;

		attr1 = slot_getattr(slot1, att, &isNull1);

		attr2 = slot_getattr(slot2, att, &isNull2);

		if (isNull1 != isNull2)
		{
			result = false;		/* one null and one not; they aren't equal */
			break;
		}

		if (isNull1)
			continue;			/* both are null, treat as equal */

		/* Apply the type-specific equality function */

		if (!DatumGetBool(FunctionCall2(&eqfunctions[i],
										attr1, attr2)))
		{
			result = false;		/* they aren't equal */
			break;
		}
	}

	MemoryContextSwitchTo(oldContext);

	return result;
}

/*
 * MemoryContextResetAndDeleteChildren
 *		Release all space allocated within a context and delete all
 *		its descendants.
 *
 * This is a common combination case where we want to preserve the
 * specific context but get rid of absolutely everything under it.
 */
void
MemoryContextResetAndDeleteChildren(MemoryContext context)
{
	AssertArg(MemoryContextIsValid(context));

	MemoryContextDeleteChildren(context);
	MemoryContextReset(context);
}

/*
 * ReScanExprContext
 *
 *		Reset an expression context in preparation for a rescan of its
 *		plan node.  This requires calling any registered shutdown callbacks,
 *		since any partially complete set-returning-functions must be canceled.
 *
 * Note we make no assumption about the caller's memory context.
 */
void
ReScanExprContext(ExprContext *econtext)
{
	/* Call any registered callbacks */
	ShutdownExprContext(econtext, true);
	/* And clean up the memory used */
	MemoryContextReset(econtext->ecxt_per_tuple_memory);
}

/*
 * Transform a tuple into a Python dict object.
 */
PyObject *
PLyDict_FromTuple(PLyTypeInfo *info, HeapTuple tuple, TupleDesc desc)
{
	PyObject   *volatile dict;
	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
	MemoryContext oldcontext = CurrentMemoryContext;
	int			i;

	if (info->is_rowtype != 1)
		elog(ERROR, "PLyTypeInfo structure describes a datum");

	dict = PyDict_New();
	if (dict == NULL)
		PLy_elog(ERROR, "could not create new dictionary");

	PG_TRY();
	{
		/*
		 * Do the work in the scratch context to avoid leaking memory from the
		 * datatype output function calls.
		 */
		MemoryContextSwitchTo(exec_ctx->scratch_ctx);
		for (i = 0; i < info->in.r.natts; i++)
		{
			char	   *key;
			Datum		vattr;
			bool		is_null;
			PyObject   *value;

			if (desc->attrs[i]->attisdropped)
				continue;

			key = NameStr(desc->attrs[i]->attname);
			vattr = heap_getattr(tuple, (i + 1), desc, &is_null);

			if (is_null || info->in.r.atts[i].func == NULL)
				PyDict_SetItemString(dict, key, Py_None);
			else
			{
				value = (info->in.r.atts[i].func) (&info->in.r.atts[i], vattr);
				PyDict_SetItemString(dict, key, value);
				Py_DECREF(value);
			}
		}
		MemoryContextSwitchTo(oldcontext);
		MemoryContextReset(exec_ctx->scratch_ctx);
	}
	PG_CATCH();
	{
		MemoryContextSwitchTo(oldcontext);
		Py_DECREF(dict);
		PG_RE_THROW();
	}
	PG_END_TRY();

	return dict;
}

static int
pg_callable_func(lua_State *L)
{
	MemoryContext m;
	int i;
	FunctionCallInfoData fcinfo;
	Lua_pgfunc *fi;

	fi = (Lua_pgfunc *) lua_touserdata(L, lua_upvalueindex(1));

	InitFunctionCallInfoData(fcinfo, &fi->fi, fi->numargs, InvalidOid, NULL, NULL);

	if(tmpcontext_usage> RESET_CONTEXT_AFTER ){
		MemoryContextReset(tmpcontext);
		tmpcontext_usage = 0;
	}
	++tmpcontext_usage;

	m = MemoryContextSwitchTo(tmpcontext);

	for (i=0; i<fi->numargs; ++i){
		fcinfo.arg[i] = luaP_todatum(L, fi->argtypes[i], 0, &fcinfo.argnull[i], i+1);
	}

	if(!fi->options.only_internal && fi->options.throwable){
		SPI_push();
		PG_TRY();
		{
			Datum d = FunctionCallInvoke(&fcinfo);
			MemoryContextSwitchTo(m);
			if (fcinfo.isnull) {
				lua_pushnil(L);
			} else {
				luaP_pushdatum(L, d, fi->prorettype);
			}
			SPI_pop();
		}
		PG_CATCH();
		{
			lua_pop(L, lua_gettop(L));
			push_spi_error(L, m); /*context switch to m inside push_spi_error*/
			SPI_pop();
			return lua_error(L);
		}PG_END_TRY();
	}else{
		Datum d = FunctionCallInvoke(&fcinfo);
		MemoryContextSwitchTo(m);
		if (fcinfo.isnull) {
			lua_pushnil(L);
		} else {
			luaP_pushdatum(L, d, fi->prorettype);
		}
	}

	return 1;
}

/*
 * MemoryContextResetChildren
 *		Release all space allocated within a context's descendants,
 *		but don't delete the contexts themselves.  The named context
 *		itself is not touched.
 */
void
MemoryContextResetChildren(MemoryContext context)
{
	MemoryContext child;

	AssertArg(MemoryContextIsValid(context));

	for (child = context->firstchild; child != NULL; child = child->nextchild)
		MemoryContextReset(child);
}

/*
 * Reset all the avc caches
 */
static void
sepgsql_avc_reset(void)
{
	MemoryContextReset(avc_mem_cxt);

	memset(avc_slots, 0, sizeof(List *) * AVC_NUM_SLOTS);
	avc_num_caches = 0;
	avc_lru_hint = 0;
	avc_unlabeled = NULL;
}

/*
 * Return the offset of the first index entry that is consistent with
 * the search key after offset 'n' in the current page. If there are
 * no more consistent entries, return InvalidOffsetNumber.
 * Page should be locked....
 */
static OffsetNumber
gistfindnext(IndexScanDesc scan, OffsetNumber n, ScanDirection dir)
{
	OffsetNumber maxoff;
	IndexTuple	it;
	GISTScanOpaque so;
	MemoryContext oldcxt;
	Page		p;

	so = (GISTScanOpaque) scan->opaque;
	p = BufferGetPage(so->curbuf);
	maxoff = PageGetMaxOffsetNumber(p);

	/*
	 * Make sure we're in a short-lived memory context when we invoke a
	 * user-supplied GiST method in gistindex_keytest(), so we don't leak
	 * memory
	 */
	oldcxt = MemoryContextSwitchTo(so->tempCxt);

	/*
	 * If we modified the index during the scan, we may have a pointer to a
	 * ghost tuple, before the scan.  If this is the case, back up one.
	 */
	if (so->flags & GS_CURBEFORE)
	{
		so->flags &= ~GS_CURBEFORE;
		n = OffsetNumberPrev(n);
	}

	while (n >= FirstOffsetNumber && n <= maxoff)
	{
		it = (IndexTuple) PageGetItem(p, PageGetItemId(p, n));
		if (gistindex_keytest(it, scan, n))
			break;

		if (ScanDirectionIsBackward(dir))
			n = OffsetNumberPrev(n);
		else
			n = OffsetNumberNext(n);
	}

	MemoryContextSwitchTo(oldcxt);
	MemoryContextReset(so->tempCxt);

	/*
	 * If we found a matching entry, return its offset; otherwise return
	 * InvalidOffsetNumber to inform the caller to go to the next page.
	 */
	if (n >= FirstOffsetNumber && n <= maxoff)
		return n;
	else
		return InvalidOffsetNumber;
}

/*
 * Reset a BrinMemTuple to initial state
 */
void
brin_memtuple_initialize(BrinMemTuple *dtuple, BrinDesc *brdesc)
{
	int			i;

	MemoryContextReset(dtuple->bt_context);
	for (i = 0; i < brdesc->bd_tupdesc->natts; i++)
	{
		dtuple->bt_columns[i].bv_allnulls = true;
		dtuple->bt_columns[i].bv_hasnulls = false;
	}
}

/**
 * Reset the context.
 */
static void ResetPMJC(PartitionJoinMutatorContext *ctx)
{
	ctx->safeToConvert = true;
	ctx->partitionOid = InvalidOid;
	ctx->partitionVarno = 0;
	ctx->partitionInfo = NULL;
	ctx->partitionKeySet = NULL;
	ctx->joinKeySet = NULL;
	ctx->outerPlan = NULL;
	ctx->outerTargetList = NULL;

	MemoryContextReset(ctx->localCtx);
}