Why is there an empty table if you don't add wait（）?

[#1]

Hello everyone, 

kz = dthz << get rows where( Is Missing( :USL ) & Is Missing( :LSL ) );
dthz << delete rows( kz );
dthz << New Column( "FAIx", "character" );
dthz << begin data update;
dthz:FAIx << set formula( If( Col Number( :FAI#, :FAI# ) > 1, :FAI# || "_" || Char( Col Rank( :FAI#, :FAI# ) ), :FAI# ) );
dthz << end data update;

Wait( 1 );//have to add
dt2 = dt1 << Join(
	With( dthz ),
	Select( all ),
	SelectWith( :FAIx ),
	By Matching Columns( :FAI# = :FAI#，:USL = :USL, :LSL = :LSL ),
	Drop multiples( 0, 0 ),
	Include Nonmatches( 1, 0 ),
	Preserve main table order( 1 ),
	Output Table( "221" ),
	invisible
);
wait(1);

The following code, before and after the addition of wait () to run normally; otherwise the back of the join this statement will be linked to the empty table reported error, saying that this is empty, what is the reason ah, sometimes normal, most of the time the empty table.

[#2 · Accepted solution]

Formulas do need time to finish evaluation and maybe end data update doesn't force the wait for evaluation to happen. Usually you want to use << Run Formulas if you wish to wait for formula evaluation to finish instead of wait(). In other cases (not formulas), most of the time, using wait(0) is enough

[#3]

Thanks. Got it.