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Support FETCH FIRST WITH TIES

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WITH TIES is an option to the FETCH FIRST N ROWS clause (the SQL
standard's spelling of LIMIT), where you additionally get rows that
compare equal to the last of those N rows by the columns in the
mandatory ORDER BY clause.

There was a proposal by Andrew Gierth to implement this functionality in
a more powerful way that would yield more features, but the other patch
had not been finished at this time, so we decided to use this one for
now in the spirit of incremental development.

Author: Surafel Temesgen <surafel3000@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Discussion: https://postgr.es/m/CALAY4q9ky7rD_A4vf=FVQvCGngm3LOes-ky0J6euMrg=_Se+ag@mail.gmail.com
Discussion: https://postgr.es/m/87o8wvz253.fsf@news-spur.riddles.org.uk
This commit is contained in:
Alvaro Herrera
2020-04-07 16:22:13 -04:00
parent 26a944cf29
commit 357889eb17
25 changed files with 616 additions and 75 deletions
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15
src/backend/parser/parse_clause.c
View File

@ -1745,7 +1745,8 @@ transformWhereClause(ParseState *pstate, Node *clause,
*/
Node *
transformLimitClause(ParseState *pstate, Node *clause,
ParseExprKind exprKind, const char *constructName)
ParseExprKind exprKind, const char *constructName,
LimitOption limitOption)
{
Node *qual;
@ -1759,6 +1760,18 @@ transformLimitClause(ParseState *pstate, Node *clause,
/* LIMIT can't refer to any variables of the current query */
checkExprIsVarFree(pstate, qual, constructName);
/*
* Don't allow NULLs in FETCH FIRST .. WITH TIES. This test is ugly and
* extremely simplistic, in that you can pass a NULL anyway by hiding it
* inside an expression -- but this protects ruleutils against emitting an
* unadorned NULL that's not accepted back by the grammar.
*/
if (exprKind == EXPR_KIND_LIMIT && limitOption == LIMIT_OPTION_WITH_TIES &&
IsA(clause, A_Const) && ((A_Const *) clause)->val.type == T_Null)
ereport(ERROR,
(errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
errmsg("row count cannot be NULL in FETCH FIRST ... WITH TIES clause")));
return qual;
}