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Improve parser's one-extra-token lookahead mechanism. 

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.
This commit is contained in:
Tom Lane 2015-02-24 17:53:42 -05:00
parent 23a78352c0
commit d809fd0008
8 changed files with 173 additions and 113 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

35
src/backend/parser/gram.y
View File

@ -633,9 +633,9 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
/*
* The grammar thinks these are keywords, but they are not in the kwlist.h
* list and so can never be entered directly. The filter in parser.c
* creates these tokens when required.
* creates these tokens when required (based on looking one token ahead).
*/
%token NULLS_FIRST NULLS_LAST WITH_ORDINALITY WITH_TIME
%token NULLS_LA WITH_LA
/* Precedence: lowest to highest */
@ -873,6 +873,7 @@ CreateRoleStmt:
opt_with: WITH {}
| WITH_LA {}
| /*EMPTY*/ {}
;
@ -6673,8 +6674,8 @@ opt_asc_desc: ASC { $$ = SORTBY_ASC; }
| /*EMPTY*/ { $$ = SORTBY_DEFAULT; }
;
opt_nulls_order: NULLS_FIRST { $$ = SORTBY_NULLS_FIRST; }
| NULLS_LAST { $$ = SORTBY_NULLS_LAST; }
opt_nulls_order: NULLS_LA FIRST_P { $$ = SORTBY_NULLS_FIRST; }
| NULLS_LA LAST_P { $$ = SORTBY_NULLS_LAST; }
| /*EMPTY*/ { $$ = SORTBY_NULLS_DEFAULT; }
;
@ -8923,7 +8924,7 @@ AlterTSDictionaryStmt:
;
AlterTSConfigurationStmt:
ALTER TEXT_P SEARCH CONFIGURATION any_name ADD_P MAPPING FOR name_list WITH any_name_list
ALTER TEXT_P SEARCH CONFIGURATION any_name ADD_P MAPPING FOR name_list any_with any_name_list
{
AlterTSConfigurationStmt *n = makeNode(AlterTSConfigurationStmt);
n->cfgname = $5;
@ -8933,7 +8934,7 @@ AlterTSConfigurationStmt:
n->replace = false;
$$ = (Node*)n;
}
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING FOR name_list WITH any_name_list
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING FOR name_list any_with any_name_list
{
AlterTSConfigurationStmt *n = makeNode(AlterTSConfigurationStmt);
n->cfgname = $5;
@ -8943,7 +8944,7 @@ AlterTSConfigurationStmt:
n->replace = false;
$$ = (Node*)n;
}
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING REPLACE any_name WITH any_name
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING REPLACE any_name any_with any_name
{
AlterTSConfigurationStmt *n = makeNode(AlterTSConfigurationStmt);
n->cfgname = $5;
@ -8953,7 +8954,7 @@ AlterTSConfigurationStmt:
n->replace = true;
$$ = (Node*)n;
}
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING FOR name_list REPLACE any_name WITH any_name
| ALTER TEXT_P SEARCH CONFIGURATION any_name ALTER MAPPING FOR name_list REPLACE any_name any_with any_name
{
AlterTSConfigurationStmt *n = makeNode(AlterTSConfigurationStmt);
n->cfgname = $5;
@ -8981,6 +8982,11 @@ AlterTSConfigurationStmt:
}
;
/* Use this if TIME or ORDINALITY after WITH should be taken as an identifier */
any_with: WITH {}
| WITH_LA {}
;
/*****************************************************************************
*
@ -9891,6 +9897,8 @@ simple_select:
* AS (query) [ SEARCH or CYCLE clause ]
*
* We don't currently support the SEARCH or CYCLE clause.
*
* Recognizing WITH_LA here allows a CTE to be named TIME or ORDINALITY.
*/
with_clause:
WITH cte_list
@ -9900,6 +9908,13 @@ with_clause:
$$->recursive = false;
$$->location = @1;
}
| WITH_LA cte_list
{
$$ = makeNode(WithClause);
$$->ctes = $2;
$$->recursive = false;
$$->location = @1;
}
| WITH RECURSIVE cte_list
{
$$ = makeNode(WithClause);
@ -10601,7 +10616,7 @@ opt_col_def_list: AS '(' TableFuncElementList ')' { $$ = $3; }
| /*EMPTY*/ { $$ = NIL; }
;
opt_ordinality: WITH_ORDINALITY { $$ = true; }
opt_ordinality: WITH_LA ORDINALITY { $$ = true; }
| /*EMPTY*/ { $$ = false; }
;
@ -11057,7 +11072,7 @@ ConstInterval:
;
opt_timezone:
WITH_TIME ZONE { $$ = TRUE; }
WITH_LA TIME ZONE { $$ = TRUE; }
| WITHOUT TIME ZONE { $$ = FALSE; }
| /*EMPTY*/ { $$ = FALSE; }
;

105
src/backend/parser/parser.c
View File

@ -64,13 +64,13 @@ raw_parser(const char *str)
/*
* Intermediate filter between parser and core lexer (core_yylex in scan.l).
*
* The filter is needed because in some cases the standard SQL grammar
* This filter is needed because in some cases the standard SQL grammar
* requires more than one token lookahead. We reduce these cases to one-token
* lookahead by combining tokens here, in order to keep the grammar LALR(1).
* lookahead by replacing tokens here, in order to keep the grammar LALR(1).
*
* Using a filter is simpler than trying to recognize multiword tokens
* directly in scan.l, because we'd have to allow for comments between the
* words. Furthermore it's not clear how to do it without re-introducing
* words. Furthermore it's not clear how to do that without re-introducing
* scanner backtrack, which would cost more performance than this filter
* layer does.
*
@ -84,7 +84,7 @@ base_yylex(YYSTYPE *lvalp, YYLTYPE *llocp, core_yyscan_t yyscanner)
base_yy_extra_type *yyextra = pg_yyget_extra(yyscanner);
int cur_token;
int next_token;
core_YYSTYPE cur_yylval;
int cur_token_length;
YYLTYPE cur_yylloc;
/* Get next token --- we might already have it */
@ -93,74 +93,85 @@ base_yylex(YYSTYPE *lvalp, YYLTYPE *llocp, core_yyscan_t yyscanner)
cur_token = yyextra->lookahead_token;
lvalp->core_yystype = yyextra->lookahead_yylval;
*llocp = yyextra->lookahead_yylloc;
*(yyextra->lookahead_end) = yyextra->lookahead_hold_char;
yyextra->have_lookahead = false;
}
else
cur_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
/* Do we need to look ahead for a possible multiword token? */
/*
* If this token isn't one that requires lookahead, just return it. If it
* does, determine the token length. (We could get that via strlen(), but
* since we have such a small set of possibilities, hardwiring seems
* feasible and more efficient.)
*/
switch (cur_token)
{
case NULLS_P:
cur_token_length = 5;
break;
case WITH:
cur_token_length = 4;
break;
default:
return cur_token;
}
/*
* NULLS FIRST and NULLS LAST must be reduced to one token
*/
cur_yylval = lvalp->core_yystype;
cur_yylloc = *llocp;
next_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
/*
* Identify end+1 of current token. core_yylex() has temporarily stored a
* '\0' here, and will undo that when we call it again. We need to redo
* it to fully revert the lookahead call for error reporting purposes.
*/
yyextra->lookahead_end = yyextra->core_yy_extra.scanbuf +
*llocp + cur_token_length;
Assert(*(yyextra->lookahead_end) == '\0');
/*
* Save and restore *llocp around the call. It might look like we could
* avoid this by just passing &lookahead_yylloc to core_yylex(), but that
* does not work because flex actually holds onto the last-passed pointer
* internally, and will use that for error reporting. We need any error
* reports to point to the current token, not the next one.
*/
cur_yylloc = *llocp;
/* Get next token, saving outputs into lookahead variables */
next_token = core_yylex(&(yyextra->lookahead_yylval), llocp, yyscanner);
yyextra->lookahead_token = next_token;
yyextra->lookahead_yylloc = *llocp;
*llocp = cur_yylloc;
/* Now revert the un-truncation of the current token */
yyextra->lookahead_hold_char = *(yyextra->lookahead_end);
*(yyextra->lookahead_end) = '\0';
yyextra->have_lookahead = true;
/* Replace cur_token if needed, based on lookahead */
switch (cur_token)
{
case NULLS_P:
/* Replace NULLS_P by NULLS_LA if it's followed by FIRST or LAST */
switch (next_token)
{
case FIRST_P:
cur_token = NULLS_FIRST;
break;
case LAST_P:
cur_token = NULLS_LAST;
break;
default:
/* save the lookahead token for next time */
yyextra->lookahead_token = next_token;
yyextra->lookahead_yylval = lvalp->core_yystype;
yyextra->lookahead_yylloc = *llocp;
yyextra->have_lookahead = true;
/* and back up the output info to cur_token */
lvalp->core_yystype = cur_yylval;
*llocp = cur_yylloc;
cur_token = NULLS_LA;
break;
}
break;
case WITH:
/*
* WITH TIME and WITH ORDINALITY must each be reduced to one token
*/
cur_yylval = lvalp->core_yystype;
cur_yylloc = *llocp;
next_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
/* Replace WITH by WITH_LA if it's followed by TIME or ORDINALITY */
switch (next_token)
{
case TIME:
cur_token = WITH_TIME;
break;
case ORDINALITY:
cur_token = WITH_ORDINALITY;
break;
default:
/* save the lookahead token for next time */
yyextra->lookahead_token = next_token;
yyextra->lookahead_yylval = lvalp->core_yystype;
yyextra->lookahead_yylloc = *llocp;
yyextra->have_lookahead = true;
/* and back up the output info to cur_token */
lvalp->core_yystype = cur_yylval;
*llocp = cur_yylloc;
cur_token = WITH_LA;
break;
}
break;
default:
break;
}
return cur_token;

2
src/include/parser/gramparse.h
View File

@ -46,6 +46,8 @@ typedef struct base_yy_extra_type
int lookahead_token; /* one-token lookahead */
core_YYSTYPE lookahead_yylval; /* yylval for lookahead token */
YYLTYPE lookahead_yylloc; /* yylloc for lookahead token */
char *lookahead_end; /* end of current token */
char lookahead_hold_char; /* to be put back at *lookahead_end */
/*
* State variables that belong to the grammar.

6
src/interfaces/ecpg/preproc/parse.pl
View File

@ -42,10 +42,8 @@ my %replace_token = (
# or in the block
my %replace_string = (
'WITH_TIME' => 'with time',
'WITH_ORDINALITY' => 'with ordinality',
'NULLS_FIRST' => 'nulls first',
'NULLS_LAST' => 'nulls last',
'NULLS_LA' => 'nulls',
'WITH_LA' => 'with',
'TYPECAST' => '::',
'DOT_DOT' => '..',
'COLON_EQUALS' => ':=',);

116
src/interfaces/ecpg/preproc/parser.c
View File

@ -3,11 +3,8 @@
* parser.c
* Main entry point/driver for PostgreSQL grammar
*
* Note that the grammar is not allowed to perform any table access
* (since we need to be able to do basic parsing even while inside an
* aborted transaction). Therefore, the data structures returned by
* the grammar are "raw" parsetrees that still need to be analyzed by
* analyze.c and related files.
* This should match src/backend/parser/parser.c, except that we do not
* need to bother with re-entrant interfaces.
*
*
* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
@ -29,18 +26,21 @@ static bool have_lookahead; /* is lookahead info valid? */
static int lookahead_token; /* one-token lookahead */
static YYSTYPE lookahead_yylval; /* yylval for lookahead token */
static YYLTYPE lookahead_yylloc; /* yylloc for lookahead token */
static char *lookahead_yytext; /* start current token */
static char *lookahead_end; /* end of current token */
static char lookahead_hold_char; /* to be put back at *lookahead_end */
/*
* Intermediate filter between parser and base lexer (base_yylex in scan.l).
*
* The filter is needed because in some cases the standard SQL grammar
* This filter is needed because in some cases the standard SQL grammar
* requires more than one token lookahead. We reduce these cases to one-token
* lookahead by combining tokens here, in order to keep the grammar LALR(1).
* lookahead by replacing tokens here, in order to keep the grammar LALR(1).
*
* Using a filter is simpler than trying to recognize multiword tokens
* directly in scan.l, because we'd have to allow for comments between the
* words. Furthermore it's not clear how to do it without re-introducing
* words. Furthermore it's not clear how to do that without re-introducing
* scanner backtrack, which would cost more performance than this filter
* layer does.
*/
@ -49,8 +49,10 @@ filtered_base_yylex(void)
{
int cur_token;
int next_token;
int cur_token_length;
YYSTYPE cur_yylval;
YYLTYPE cur_yylloc;
char *cur_yytext;
/* Get next token --- we might already have it */
if (have_lookahead)
@ -58,74 +60,86 @@ filtered_base_yylex(void)
cur_token = lookahead_token;
base_yylval = lookahead_yylval;
base_yylloc = lookahead_yylloc;
yytext = lookahead_yytext;
*lookahead_end = lookahead_hold_char;
have_lookahead = false;
}
else
cur_token = base_yylex();
/* Do we need to look ahead for a possible multiword token? */
/*
* If this token isn't one that requires lookahead, just return it. If it
* does, determine the token length. (We could get that via strlen(), but
* since we have such a small set of possibilities, hardwiring seems
* feasible and more efficient.)
*/
switch (cur_token)
{
case NULLS_P:
cur_token_length = 5;
break;
case WITH:
cur_token_length = 4;
break;
default:
return cur_token;
}
/*
* NULLS FIRST and NULLS LAST must be reduced to one token
*/
cur_yylval = base_yylval;
cur_yylloc = base_yylloc;
next_token = base_yylex();
/*
* Identify end+1 of current token. base_yylex() has temporarily stored a
* '\0' here, and will undo that when we call it again. We need to redo
* it to fully revert the lookahead call for error reporting purposes.
*/
lookahead_end = yytext + cur_token_length;
Assert(*lookahead_end == '\0');
/* Save and restore lexer output variables around the call */
cur_yylval = base_yylval;
cur_yylloc = base_yylloc;
cur_yytext = yytext;
/* Get next token, saving outputs into lookahead variables */
next_token = base_yylex();
lookahead_token = next_token;
lookahead_yylval = base_yylval;
lookahead_yylloc = base_yylloc;
lookahead_yytext = yytext;
base_yylval = cur_yylval;
base_yylloc = cur_yylloc;
yytext = cur_yytext;
/* Now revert the un-truncation of the current token */
lookahead_hold_char = *lookahead_end;
*lookahead_end = '\0';
have_lookahead = true;
/* Replace cur_token if needed, based on lookahead */
switch (cur_token)
{
case NULLS_P:
/* Replace NULLS_P by NULLS_LA if it's followed by FIRST or LAST */
switch (next_token)
{
case FIRST_P:
cur_token = NULLS_FIRST;
break;
case LAST_P:
cur_token = NULLS_LAST;
break;
default:
/* save the lookahead token for next time */
lookahead_token = next_token;
lookahead_yylval = base_yylval;
lookahead_yylloc = base_yylloc;
have_lookahead = true;
/* and back up the output info to cur_token */
base_yylval = cur_yylval;
base_yylloc = cur_yylloc;
cur_token = NULLS_LA;
break;
}
break;
case WITH:
/*
* WITH TIME must be reduced to one token
*/
cur_yylval = base_yylval;
cur_yylloc = base_yylloc;
next_token = base_yylex();
/* Replace WITH by WITH_LA if it's followed by TIME or ORDINALITY */
switch (next_token)
{
case TIME:
cur_token = WITH_TIME;
break;
case ORDINALITY:
cur_token = WITH_ORDINALITY;
break;
default:
/* save the lookahead token for next time */
lookahead_token = next_token;
lookahead_yylval = base_yylval;
lookahead_yylloc = base_yylloc;
have_lookahead = true;
/* and back up the output info to cur_token */
base_yylval = cur_yylval;
base_yylloc = cur_yylloc;
cur_token = WITH_LA;
break;
}
break;
default:
break;
}
return cur_token;

2
src/test/regress/expected/foreign_data.out
View File

@ -663,7 +663,7 @@ LINE 1: CREATE FOREIGN TABLE ft1 ();
CREATE FOREIGN TABLE ft1 () SERVER no_server; -- ERROR
ERROR: server "no_server" does not exist
CREATE FOREIGN TABLE ft1 () SERVER s0 WITH OIDS; -- ERROR
ERROR: syntax error at or near "WITH OIDS"
ERROR: syntax error at or near "WITH"
LINE 1: CREATE FOREIGN TABLE ft1 () SERVER s0 WITH OIDS;
^
CREATE FOREIGN TABLE ft1 (

15
src/test/regress/expected/with.out
View File

@ -2155,3 +2155,18 @@ WITH t AS (
VALUES(FALSE);
ERROR: conditional DO INSTEAD rules are not supported for data-modifying statements in WITH
DROP RULE y_rule ON y;
-- check that parser lookahead for WITH doesn't cause any odd behavior
create table foo (with baz); -- fail, WITH is a reserved word
ERROR: syntax error at or near "with"
LINE 1: create table foo (with baz);
^
create table foo (with ordinality); -- fail, WITH is a reserved word
ERROR: syntax error at or near "with"
LINE 1: create table foo (with ordinality);
^
with ordinality as (select 1 as x) select * from ordinality;
x
---
1
(1 row)

5
src/test/regress/sql/with.sql
View File

@ -956,3 +956,8 @@ WITH t AS (
)
VALUES(FALSE);
DROP RULE y_rule ON y;
-- check that parser lookahead for WITH doesn't cause any odd behavior
create table foo (with baz); -- fail, WITH is a reserved word
create table foo (with ordinality); -- fail, WITH is a reserved word
with ordinality as (select 1 as x) select * from ordinality;