Function array_subscript_transform

src/backend/utils/adt/arraysubs.c:54–164

* Finish parse analysis of a SubscriptingRef expression for an array. * * Transform the subscript expressions, coerce them to integers, * and determine the result type of the SubscriptingRef node. */

 * and determine the result type of the SubscriptingRef node.
 */
static void
array_subscript_transform(SubscriptingRef *sbsref,
						  List *indirection,
						  ParseState *pstate,
						  bool isSlice,
						  bool isAssignment)
{
	List	   *upperIndexpr = NIL;
	List	   *lowerIndexpr = NIL;
	ListCell   *idx;

	/*
	 * Transform the subscript expressions, and separate upper and lower
	 * bounds into two lists.
	 *
	 * If we have a container slice expression, we convert any non-slice
	 * indirection items to slices by treating the single subscript as the
	 * upper bound and supplying an assumed lower bound of 1.
	 */
	foreach(idx, indirection)
	{
		A_Indices  *ai = lfirst_node(A_Indices, idx);
		Node	   *subexpr;

		if (isSlice)
		{
			if (ai->lidx)
			{
				subexpr = transformExpr(pstate, ai->lidx, pstate->p_expr_kind);
				/* If it's not int4 already, try to coerce */
				subexpr = coerce_to_target_type(pstate,
												subexpr, exprType(subexpr),
												INT4OID, -1,
												COERCION_ASSIGNMENT,
												COERCE_IMPLICIT_CAST,
												-1);
				if (subexpr == NULL)
					ereport(ERROR,
							(errcode(ERRCODE_DATATYPE_MISMATCH),
							 errmsg("array subscript must have type integer"),
							 parser_errposition(pstate, exprLocation(ai->lidx))));
			}
			else if (!ai->is_slice)
			{
				/* Make a constant 1 */
				subexpr = (Node *) makeConst(INT4OID,
											 -1,
											 InvalidOid,
											 sizeof(int32),
											 Int32GetDatum(1),
											 false,
											 true); /* pass by value */
			}
			else
			{
				/* Slice with omitted lower bound, put NULL into the list */
				subexpr = NULL;
			}