Postgres: support WHERE index_predicate in the ON CONFLICT conflict target #2625

Summary

In PostgreSQL, an ON CONFLICT conflict target given as a column list may end with WHERE index_predicate, which lets a partial unique index be inferred as the arbiter. sqlparser rejects the predicate, in both the PostgreSQL and the generic dialect.

This is the conflict target's WHERE. The action's DO UPDATE SET ... WHERE condition is already supported (#733, #735).

Reproduction (sqlparser 0.63.0)

use sqlparser::{dialect::PostgreSqlDialect, parser::Parser};

let sql = "INSERT INTO t (a, b) VALUES (1, 2) ON CONFLICT (a) WHERE b > 0 DO NOTHING";
Parser::parse_sql(&PostgreSqlDialect {}, sql).unwrap();

sql parser error: Expected: DO, found: WHERE at Line: 1, Column: 52

... ON CONFLICT (a) WHERE b > 0 DO UPDATE SET b = EXCLUDED.b fails the same way. Both run on PostgreSQL (checked on 16.15):

CREATE TABLE t (a int, b int);
CREATE UNIQUE INDEX ON t (a) WHERE b > 0;
INSERT INTO t (a, b) VALUES (1, 2) ON CONFLICT (a) WHERE b > 0 DO NOTHING;
INSERT INTO t (a, b) VALUES (1, 2) ON CONFLICT (a) WHERE b > 0 DO UPDATE SET b = EXCLUDED.b;

Grammar

From https://www.postgresql.org/docs/current/sql-insert.html:

ON CONFLICT [ conflict_target ] conflict_action

where conflict_target can be one of:

    ( { index_column_name | ( index_expression ) } [ COLLATE collation ] [ opclass ] [, ...] ) [ WHERE index_predicate ]
    ON CONSTRAINT constraint_name

Proposed change

An optional predicate expression on the column-list conflict target, parsed after the parenthesized list and before DO, and printed in the same place. Two possible shapes:

• a new field on OnConflict, e.g. pub conflict_target_predicate: Option<Expr>; or
• ConflictTarget::Columns as a struct variant, Columns { columns: Vec<Ident>, predicate: Option<Expr> }.

The struct variant would also leave room for the ( index_expression ) form requested in #1977. This issue does not cover that form, nor COLLATE / opclass. I'm happy to follow the maintainers' preference and send a PR.