liquibase-core/src/main/java/liquibase/database/AbstractJdbcDatabase.java
protected String getConnectionSchemaName() {
        if (connection == null) {
            return null;
        }
        if (connection instanceof OfflineConnection) {
            return ((OfflineConnection) connection).getSchema();
        }

        try {
            SqlStatement currentSchemaStatement = getConnectionSchemaNameCallStatement();
            return ExecutorService.getInstance().getExecutor(this).
                    queryForObject(currentSchemaStatement, String.class);
        } catch (Exception e) {
            Scope.getCurrentScope().getLog(getClass()).info(LogType.LOG, "Error getting default schema", e);
        }
        return null;
    }
protected String getConnectionSchemaName() {
        if (connection == null) {
            return null;
        }
        if (connection instanceof OfflineConnection) {
            return ((OfflineConnection) connection).getSchema();
        }

        try {
            SqlStatement currentSchemaStatement = getConnectionSchemaNameCallStatement();
            return ExecutorService.getInstance().getExecutor(this).
                    queryForObject(currentSchemaStatement, String.class);
        } catch (Exception e) {
            Scope.getCurrentScope().getLog(getClass()).info(LogType.LOG, "Error getting default schema", e);
        }
        return null;
    }
Overwrite this method to get the default schema name for the connection.
If you only need to change the statement that obtains the current schema then override
@see AbstractJdbcDatabase#getConnectionSchemaNameCallStatement()