"Consing up" is an informal term derived from Lisp terminology. In this context, it means dynamically creating (allocating and constructing) a new NullTest node. Instead of reusing an existing node, the code allocates a fresh node—using PostgreSQL’s memory allocation (palloc) functions—and fills in its fields (like the generation expression, null test type, etc.). This new node is then prepared (compiled) for execution with ExecPrepareExpr and used to evaluate whether the generated expression returns a non-NULL value, thus enforcing the NOT NULL constraint on virtual generated columns.
+ * We implement this by consing up a NullTest node for each virtual trivial question. I googled, and still found any explanation of the word "consing up".
