Fixing shift/reduce conflicts in Bison
From PostgreSQL wiki
Postgres development rules forbid shift/reduce conflicts in the main grammar (and the other conflicts bison produces - reduce/reduce conflicts - are even worse). Often if you're making a grammar change, you can introduce a shift/reduce conflict to the grammar that needs to be fixed. As Tom Lane explains here, shift/reduce conflicts can often be fixed by "unfactoring" the grammar a little. (For more context see the original discussion on the -hackers mailing list.)
For those interested in a bit more of the theory of parsers, most compiler construction books cover them to some degree, including the venerable if a bit dated Dragon Book.
> > bison -v doesn't show anything useful beyond saying that there is one > > shift/reduce conflict. The gram.output is 10MB, which doesn't help me > > much (I'm still trying to make sense of it). Well, you need to learn a bit more about bison I think. The complaint is State 1135 conflicts: 1 shift/reduce so we look at state 1135, which says: state 1135 241 alter_table_cmd: ADD_P . opt_column columnDef 251 | ADD_P . TableConstraint CHECK shift, and go to state 1698 COLUMN shift, and go to state 1742 CONSTRAINT shift, and go to state 1699 EXCLUSION shift, and go to state 1700 FOREIGN shift, and go to state 1701 PRIMARY shift, and go to state 1702 UNIQUE shift, and go to state 1703 EXCLUSION [reduce using rule 887 (opt_column)] $default reduce using rule 887 (opt_column) TableConstraint go to state 1743 ConstraintElem go to state 1705 opt_column go to state 1744 This is the state immediately after scanning "ADD" in an ALTER TABLE command, and what it's unhappy about is that it has two different things to do if the next token is EXCLUSION. (The dot in the productions indicates "where we are", and the square brackets mark the unreachable action.) If you check the other mentioned states it becomes clear that the state-1700 path leads to deciding that EXCLUSION begins a TableConstraint, while rule 887 is the "empty" alternative for opt_column, and is what would have to be done next if EXCLUSION is a column name beginning a ColumnDef. So the difficulty is that it can't be sure whether EXCLUSION is a column name without looking one token past EXCLUSION, but it has to decide whether to eliminate COLUMN before it can look ahead past EXCLUSION. This is a pretty common difficulty with empty-producing productions. The usual way around it is to eliminate the empty production by making the calling production a bit more redundant. In this case, we can fix it by replacing alter_table_cmd: ADD_P opt_column columnDef with two productions alter_table_cmd: ADD_P columnDef | ADD_P COLUMN columnDef The reason this fixes it is that now the parser does not have to make a shift-reduce decision while EXCLUSION is the next token: it's just going to shift all the time, and it only has to reduce once EXCLUSION is the current token and it can see the next one as lookahead. (In which case, it will reduce EXCLUSION to ColId and proceed with the its-a-ColumnDef path, only if the next token isn't "(" or "USING".) Another way to think about it is that we are forcing bison to split this one state into two, but I find it easier to understand how to fix the problem by looking for ways to postpone the reduce decision.