Function ExecComputeStoredGenerated

src/backend/executor/nodeModifyTable.c:324–440

* Compute stored generated columns for a tuple */

 * Compute stored generated columns for a tuple
 */
void
ExecComputeStoredGenerated(ResultRelInfo *resultRelInfo,
						   EState *estate, TupleTableSlot *slot,
						   CmdType cmdtype)
{
	Relation	rel = resultRelInfo->ri_RelationDesc;
	TupleDesc	tupdesc = RelationGetDescr(rel);
	int			natts = tupdesc->natts;
	MemoryContext oldContext;
	Datum	   *values;
	bool	   *nulls;

	Assert(tupdesc->constr && tupdesc->constr->has_generated_stored);

	/*
	 * If first time through for this result relation, build expression
	 * nodetrees for rel's stored generation expressions.  Keep them in the
	 * per-query memory context so they'll survive throughout the query.
	 */
	if (resultRelInfo->ri_GeneratedExprs == NULL)
	{
		oldContext = MemoryContextSwitchTo(estate->es_query_cxt);

		resultRelInfo->ri_GeneratedExprs =
			(ExprState **) palloc(natts * sizeof(ExprState *));
		resultRelInfo->ri_NumGeneratedNeeded = 0;

		for (int i = 0; i < natts; i++)
		{
			if (TupleDescAttr(tupdesc, i)->attgenerated == ATTRIBUTE_GENERATED_STORED)
			{
				Expr	   *expr;

				/*
				 * If it's an update and the current column was not marked as
				 * being updated, then we can skip the computation.  But if
				 * there is a BEFORE ROW UPDATE trigger, we cannot skip
				 * because the trigger might affect additional columns.
				 */
				if (cmdtype == CMD_UPDATE &&
					!(rel->trigdesc && rel->trigdesc->trig_update_before_row) &&
					!bms_is_member(i + 1 - FirstLowInvalidHeapAttributeNumber,
								   ExecGetExtraUpdatedCols(resultRelInfo, estate)))
				{
					resultRelInfo->ri_GeneratedExprs[i] = NULL;
					continue;
				}

				expr = (Expr *) build_column_default(rel, i + 1);
				if (expr == NULL)
					elog(ERROR, "no generation expression found for column number %d of table \"%s\"",
						 i + 1, RelationGetRelationName(rel));

				resultRelInfo->ri_GeneratedExprs[i] = ExecPrepareExpr(expr, estate);
				resultRelInfo->ri_NumGeneratedNeeded++;
			}
		}