On 11/17/15, Yuri <yuri at rawbw.com> wrote: > This message always leaves the user wondering: "Which constraint?" > > How hard is it to add this information to the message? Is this a matter > of memorizing the ID of the constraint, and then printing its name in > the message? >
It is a substantial change (basically a complete rewrite of the entire foreign key constraint mechanism) which would negatively impact both space and performance. The current foreign key constraint mechanism uses a single counter. As constraints are violated, the counter increments, and as constraints are resolved the counter decrements. At the end, if the counter is greater than zero then a "foreign key constraint" error is issued. To provide information about which constraint(s) failed, it would be necessary to have a bag (a list or hash table or an associative array) of all the constraints that have been violated and then remove elements from the bag as constraints are resolved. A bag takes more run-time memory than a single counter. (Maybe a lot more, depending on how many elements it holds.) Adding an element to a bag takes more time than incrementing a counter. (In particular, adding an element to a bag probably involves one or more calls to malloc().) Removing an element from a bag takes more time than decrementing a counter. -- D. Richard Hipp drh at sqlite.org
