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There are a couple of places in our grammar that fail to be strict LALR(1), by requiring more than a single token of lookahead to decide what to do. Up to now we've dealt with that by using a filter between the lexer and parser that merges adjacent tokens into one in the places where two tokens of lookahead are necessary. But that creates a number of user-visible anomalies, for instance that you can't name a CTE "ordinality" because "WITH ordinality AS ..." triggers folding of WITH and ORDINALITY into one token. I realized that there's a better way. In this patch, we still do the lookahead basically as before, but we never merge the second token into the first; we replace just the first token by a special lookahead symbol when one of the lookahead pairs is seen. This requires a couple extra productions in the grammar, but it involves fewer special tokens, so that the grammar tables come out a bit smaller than before. The filter logic is no slower than before, perhaps a bit faster. I also fixed the filter logic so that when backing up after a lookahead, the current token's terminator is correctly restored; this eliminates some weird behavior in error message issuance, as is shown by the one change in existing regression test outputs. I believe that this patch entirely eliminates odd behaviors caused by lookahead for WITH. It doesn't really improve the situation for NULLS followed by FIRST/LAST unfortunately: those sequences still act like a reserved word, even though there are cases where they should be seen as two ordinary identifiers, eg "SELECT nulls first FROM ...". I experimented with additional grammar hacks but couldn't find any simple solution for that. Still, this is better than before, and it seems much more likely that we *could* somehow solve the NULLS case on the basis of this filter behavior than the previous one. |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.