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Remove precedence hacks no longer needed without postfix operators.

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It's no longer necessary to assign explicit precedences to GENERATED,
NULL_P, PRESERVE, or STRIP_P.

Actually, we don't need to assign precedence to IDENT either; that was
really just there to govern the behavior of target_el's "a_expr IDENT"
production, which no longer ends with that terminal.  However, it seems
like a good idea to continue to do so, because it provides a reference
point for a precedence level that we can assign to other unreserved
keywords that lack a natural precedence level.

Research by Peter Eisentraut and John Naylor; comment rewrite by me.

Discussion: https://postgr.es/m/38ca86db-42ab-9b48-2902-337a0d6b8311@2ndquadrant.com
This commit is contained in:
Tom Lane 2020-09-19 15:11:26 -04:00
parent 8354e7b27e
commit 28a61fc6c5
1 changed files with 15 additions and 19 deletions
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34
src/backend/parser/gram.y
View File

@ -744,22 +744,15 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
%nonassoc BETWEEN IN_P LIKE ILIKE SIMILAR NOT_LA
%nonassoc ESCAPE /* ESCAPE must be just above LIKE/ILIKE/SIMILAR */
/*
* To support target_el without AS, we must give IDENT an explicit priority
* between ESCAPE and Op. We can safely assign the same priority to
* various unreserved keywords as needed to resolve ambiguities (this can't
* have any bad effects since obviously the keywords will still behave the
* same as if they weren't keywords). We need to do this:
* for PARTITION, RANGE, ROWS, GROUPS to support opt_existing_window_name;
* for RANGE, ROWS, GROUPS so that they can follow a_expr without creating
* postfix-operator problems;
* for GENERATED so that it can follow b_expr;
* and for NULL so that it can follow b_expr in ColQualList without creating
* postfix-operator problems.
* To support target_el without AS, it used to be necessary to assign IDENT an
* explicit precedence just less than Op. While that's not really necessary
* since we removed postfix operators, it's still helpful to do so because
* there are some other unreserved keywords that need precedence assignments.
* If those keywords have the same precedence as IDENT then they clearly act
* the same as non-keywords, reducing the risk of unwanted precedence effects.
*
* To support CUBE and ROLLUP in GROUP BY without reserving them, we give them
* an explicit priority lower than '(', so that a rule with CUBE '(' will shift
* rather than reducing a conflicting rule that takes CUBE as a function name.
* Using the same precedence as IDENT seems right for the reasons given above.
* We need to do this for PARTITION, RANGE, ROWS, and GROUPS to support
* opt_existing_window_name (see comment there).
*
* The frame_bound productions UNBOUNDED PRECEDING and UNBOUNDED FOLLOWING
* are even messier: since UNBOUNDED is an unreserved keyword (per spec!),
@ -769,9 +762,14 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
* appear to cause UNBOUNDED to be treated differently from other unreserved
* keywords anywhere else in the grammar, but it's definitely risky. We can
* blame any funny behavior of UNBOUNDED on the SQL standard, though.
*
* To support CUBE and ROLLUP in GROUP BY without reserving them, we give them
* an explicit priority lower than '(', so that a rule with CUBE '(' will shift
* rather than reducing a conflicting rule that takes CUBE as a function name.
* Using the same precedence as IDENT seems right for the reasons given above.
*/
%nonassoc UNBOUNDED /* ideally should have same precedence as IDENT */
%nonassoc IDENT GENERATED NULL_P PARTITION RANGE ROWS GROUPS PRECEDING FOLLOWING CUBE ROLLUP
%nonassoc UNBOUNDED /* ideally would have same precedence as IDENT */
%nonassoc IDENT PARTITION RANGE ROWS GROUPS PRECEDING FOLLOWING CUBE ROLLUP
%left Op OPERATOR /* multi-character ops and user-defined operators */
%left '+' '-'
%left '*' '/' '%'
@ -792,8 +790,6 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
* left-associativity among the JOIN rules themselves.
*/
%left JOIN CROSS LEFT FULL RIGHT INNER_P NATURAL
/* kluge to keep xml_whitespace_option from causing shift/reduce conflicts */
%right PRESERVE STRIP_P
%%