如何在Firebird执行块中交互式设置declare变量值？

我可以运行以下SQL实现Firebird数据库的伪全文搜索：

--伪全文搜索实现
--参考全表查找逻辑实现
--TODO: 搜索字符串（及匹配条件）应在执行时交互式设置，而非硬编码

execute block
returns (
    table_name varchar(40),
    table_desc varchar(63),
    column_name varchar(40),
    column_desc varchar(63),
    drilldown_SQL varchar(255)
    )
as
declare search_value varchar(30) = 'search_string';
--declare search_condition varchar(30) = '=';
--declare search_condition varchar(30) ='like';
declare search_condition varchar(30) ='containing';
declare has_result boolean;
begin
    for select 
              trim(r.rdb$relation_name)
              , substring(r.rdb$description from 1 for 63)
              , trim(f.rdb$field_name)
              , substring(f.rdb$description from 1 for 63)
              , 'select '||trim(f.rdb$field_name)|| ', '||trim(r.rdb$relation_name)||
                '.* from ' || trim(r.rdb$relation_name) || 
                ' where ' || trim(f.rdb$field_name) || ' ' || :search_condition || ' ''' || :search_value || ''''
        from rdb$relation_fields f
        join rdb$relations r on f.rdb$relation_name = r.rdb$relation_name
        and r.rdb$view_blr is null
        and (r.rdb$system_flag is null or r.rdb$system_flag = 0)
        order by r.rdb$relation_name, f.rdb$field_position
        into :table_name, :table_desc, :column_name, :column_desc, :drilldown_SQL
    do
    begin
        execute statement ('select exists(select * from "' || table_name || '" where "' || column_name || '" ' || search_condition || ' ?) from rdb$database') (search_value)
            into has_result;
        if (has_result) then
            suspend;
        when any do
        begin
            /* 值无法与varchar比较，跳过 */
        end
    end
end

遇到的问题 #

• 单独执行带参数的查询语句（如select 'abc' as row1 from RDB$DATABASE where 1 like :string）时，会正常弹出参数输入提示并执行成功
• 但在上述伪全文搜索的execute block中，尝试将硬编码的搜索值改为交互式参数（即把declare search_value varchar(30) = 'search_string';改为declare search_value varchar(30) = :searchstring;）时，会触发错误**-104（未知Token :）**

运行环境限制 #

该SQL运行在ERP软件的SQL执行对话框中，仅拥有只读权限：

• 支持执行select语句和execute block
• 无法连续执行多条SQL语句（例如select * from table1;select * from table2;会报错）

内容的提问来源于stack exchange，提问作者sirtet