in both an activating statement and its associated trigger trees the first of these operators is illustrated in fig2 a which shows a flow operator the function of this operator is to move the output of operator op 1 12 to the input of operator op 2 14 as the output of operator op 1 is produced for example if op 1 is a selection operator on a table which selects rows of the table meeting a certain condition then as the rows meeting the condition are found say by scanning the table the rows are sent to the input of op 2 this permits the op 2 operator to function in parallel to the op 1 operator though of course not on the same row that op 1 is operating on fig2 a illustrates this u201c pipelining u201d operation in a timing chart which shows the activity of op 1 overlapped with the activity of op 2 0031 fig2 b and 2c illustrate the union operators the ordered union operator 16 of fig2 b forces op 2 to operate only after op 1 has completed its operations in effect serializing the op 1 op 2 operations as shown in the timing chart the parallel union operator 18 allows op 2 to operate concurrently with op 1 and assumes that op 2 has no data access conflict with op 1 as is evident from fig2 a and 2c the flow operator 10 and the parallel union operator 18 reduce the.