remapdb Database Remapping Hint

remapdb rewrites qualified database references from a source database to a target database in SELECT, DML, table DDL, and ANALYZE statements; it is gated by enable_remap_hint and can be set per session or per query.

Description

remapdb is a database-name remapping rule used by the MatrixOne query rewrite feature. It maps a source database name to a target database name, so that a qualified reference such as source_db.table resolves to target_db.table.

The rule is supplied as a JSON object under the remapdb key and can be set in two ways:

  • As the value of the remap_rewrites session variable, applying to every subsequent query in the session.

  • As an inline /*+ ... */ hint, applying to the single query that carries it.

Both forms are gated by enable_remap_hint; when that variable is off, the rule is ignored.

Syntax

{"remapdb": {"source_database": "target_database", ...}}

SET remap_rewrites = '{"remapdb": {"source_database": "target_database"}}'
/*+ {"remapdb": {"source_database": "target_database"}} */ SELECT ...

Only qualified references are rewritten. USE is not remapped, but while the current database is a remap source, unqualified names resolve against the target database. remapdb is applied before the table rewrites rules.

Arguments

remapdb takes a single value: a JSON object under the remapdb key that maps source database names to target database names.

Examples

DROP DATABASE IF EXISTS remapdb_dst;
CREATE DATABASE remapdb_dst;
USE remapdb_dst;

DROP DATABASE IF EXISTS remapdb_src;
CREATE DATABASE remapdb_src;
CREATE TABLE remapdb_dst.t (id INT);
CREATE TABLE remapdb_src.t (id INT);
INSERT INTO remapdb_dst.t VALUES (1), (2), (3);
INSERT INTO remapdb_src.t VALUES (100), (200);

SET enable_remap_hint = 1;
SET remap_rewrites = '{"remapdb": {"remapdb_src": "remapdb_dst"}}';
SELECT * FROM remapdb_src.t ORDER BY id;

SET remap_rewrites = '';
SET enable_remap_hint = 0;

DROP DATABASE remapdb_src;
DROP DATABASE remapdb_dst;

Constraints

  • remapdb names must be valid identifiers.

  • The source and destination database sets must be disjoint; chaining (a to b and b to c) and self-mapping are rejected.

  • Multiple sources may map to the same destination.

  • System databases (information_schema, mysql, system, system_metrics, and any mo_* database) cannot be used as either a source or a destination.

  • Database-level DDL (CREATE/DROP/ALTER DATABASE, USE) is never remapped.

See Also