hibernate/hibernate-orm · error · QueryException

Can't emulate json_objectagg 'with unique keys' clause.

Error message

Can't emulate json_objectagg 'with unique keys' clause.

What it means

SAP HANA lacks native json_objectagg, so Hibernate emulates it with string_agg. The emulation cannot enforce key uniqueness, so a WITH UNIQUE KEYS request is rejected with this QueryException before SQL is generated.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/dialect/function/json/HANAJsonObjectAggFunction.java:37

/**
 * SAP HANA json_objectagg function.
 */
public class HANAJsonObjectAggFunction extends JsonObjectAggFunction {

	public HANAJsonObjectAggFunction(TypeConfiguration typeConfiguration) {
		super( ",", false, typeConfiguration );
	}

	@Override
	protected void render(
			SqlAppender sqlAppender,
			JsonObjectAggArguments arguments,
			Predicate filter,
			ReturnableType<?> returnType,
			SqlAstTranslator<?> translator) {
		final boolean caseWrapper = filter != null;
		if ( arguments.uniqueKeysBehavior() == JsonObjectAggUniqueKeysBehavior.WITH ) {
			throw new QueryException( "Can't emulate json_objectagg 'with unique keys' clause." );
		}
		sqlAppender.appendSql( "'{'||string_agg(" );
		renderArgument( sqlAppender, arguments.key(), JsonNullBehavior.NULL, translator );
		sqlAppender.appendSql( "||':'||" );
		if ( caseWrapper ) {
			if ( arguments.nullBehavior() != JsonNullBehavior.ABSENT ) {
				throw new QueryException( "Can't emulate json_objectagg filter clause when using 'null on null' clause." );
			}
			translator.getCurrentClauseStack().push( Clause.WHERE );
			sqlAppender.appendSql( "case when " );
			filter.accept( translator );
			translator.getCurrentClauseStack().pop();
			sqlAppender.appendSql( " then " );
			renderArgument( sqlAppender, arguments.value(), arguments.nullBehavior(), translator );
			sqlAppender.appendSql( " else null end)" );
		}
		else {
			renderArgument( sqlAppender, arguments.value(), arguments.nullBehavior(), translator );

View on GitHub (pinned to fad1729dce)

Solutions

1. Remove WITH UNIQUE KEYS from the json_objectagg call on HANA.
2. Deduplicate key/value pairs in a subquery or in Java before the aggregate.
3. Use a native HANA query when unique-key enforcement must stay in the database.
4. Validate keys for uniqueness in application code after the aggregate returns.

Example fix

// before
select json_objectagg(k value v with unique keys) from Entity e

// after
select json_objectagg(k value v) from Entity e

boolean hana = session.getJdbcServices().getDialect() instanceof org.hibernate.dialect.HANADialect;
if (hana && withUniqueKeys) {
    throw new UnsupportedOperationException("HANA json_objectagg cannot enforce WITH UNIQUE KEYS; deduplicate first");
}

try {
    return session.createQuery(hql).getSingleResult();
} catch (org.hibernate.QueryException e) {
    if (e.getMessage() != null && e.getMessage().contains("with unique keys")) {
        // Retry without WITH UNIQUE KEYS after deduplicating in a subquery.
        return session.createQuery(fallbackHql).getSingleResult();
    }
    throw e;
}

When it happens

Trigger: An HQL query on HANA calls json_objectagg(key VALUE value WITH UNIQUE KEYS). The check arguments.uniqueKeysBehavior() == JsonObjectAggUniqueKeysBehavior.WITH fires and rendering stops.

Common situations: Queries ported from Oracle or MySQL 8, where WITH UNIQUE KEYS is supported. Mappings that produce duplicate keys per group and rely on the database to drop duplicates.

Related errors

• Can't emulate json_objectagg filter clause when using 'null
• Can't emulate json_objectagg 'with unique keys' clause.
• Can't emulate json_arrayagg filter clause when using 'null o
• Can't emulate json_objectagg filter clause when using 'null
• Can't emulate json_objectagg 'with unique keys' clause.

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/f2889baa4b865ac0. Report an issue: GitHub.