Getting around MySQL “Can't reopen table” error

Question

I'm currently busy implementing a filter of sorts for which I need to generate an INNER JOIN clausse for every "tag" to filter on.

The problem is that after a whole bunch of SQL, I have a table that contains all the information I need to make my selection, but I need it again for every generated INNER JOIN

This basically looks like:

SELECT
    *
FROM search
INNER JOIN search f1 ON f1.baseID = search.baseID AND f1.condition = condition1
INNER JOIN search f2 ON f2.baseID = search.baseID AND f2.condition = condition2
...
INNER JOIN search fN ON fN.baseID = search.baseID AND fN.condition = conditionN

This works but I would much prefer the "search" table to be temporary (it can be several orders of magnitude smaller if it isn't a normal table) but that gives me a very annoying error: Can't reopen table

Some research leads me to this bug report but the folks over at MySQL don't seem to care that such a basic feature (using a table more than once) does not work with temporary tables. I'm running into a lot of scalability problems with this issue.

Is there any viable workaround that does not require me to manage potentially lots of temporary but very real tables or make me maintain a huge table with all the data in it?

Kind regards, Kris

[additional]

The GROUP_CONCAT answer does not work in my situation because my conditions are multiple columns in specific order, it would make ORs out of what I need to be ANDs. However, It did help me solve an earlier problem so now the table, temp or not, is no longer required. We were just thinking too generic for our problem. The entire application of filters has now been brought back from around a minute to well under a quarter of a second.

Answer 1

Right, the MySQL docs say: "You cannot refer to a TEMPORARY table more than once in the same query."

Here's an alternative query that should find the same rows, although all the conditions of matching rows won't be in separate columns, they'll be in a comma-separated list.

SELECT f1.baseID, GROUP_CONCAT(f1.condition)
FROM search f1
WHERE f1.condition IN (<condition1>, <condition2>, ... <conditionN>)
GROUP BY f1.baseID
HAVING COUNT(*) = <N>;

Answer 2

A simple solution is to duplicate the temporary table. Works well if the table is relatively small, which is often the case with temporary tables.

Answer 3

I got around this by creating a permanent "temporary" table and suffixing the SPID (sorry, i'm from SQL Server land) to the table name, to make a unique table name. Then creating dynamic SQL statements to create the queries. If anything bad happens, the table will be dropped and recreated.

I'm hoping for a better option. C'mon, MySQL Devs. The 'bug'/'feature request' has been open since 2008! Seems like all the 'bugs' 've encountered are in the same boat.

select concat('ReviewLatency', CONNECTION_ID()) into @tablename;

#Drop "temporary" table if it exists
set @dsql=concat('drop table if exists ', @tablename, ';');
PREPARE QUERY1 FROM @dsql;
EXECUTE QUERY1;
DEALLOCATE PREPARE QUERY1;

#Due to MySQL bug not allowing multiple queries in DSQL, we have to break it up...
#Also due to MySQL bug, you cannot join a temporary table to itself,
#so we create a real table, but append the SPID to it for uniqueness.
set @dsql=concat('
create table ', @tablename, ' (
    `EventUID` int(11) not null,
    `EventTimestamp` datetime not null,
    `HasAudit` bit not null,
    `GroupName` varchar(255) not null,
    `UserID` int(11) not null,
    `EventAuditUID` int(11) null,
    `ReviewerName` varchar(255) null,
    index `tmp_', @tablename, '_EventUID` (`EventUID` asc),
    index `tmp_', @tablename, '_EventAuditUID` (`EventAuditUID` asc),
    index `tmp_', @tablename, '_EventUID_EventTimestamp` (`EventUID`, `EventTimestamp`)
) ENGINE=MEMORY;');
PREPARE QUERY2 FROM @dsql;
EXECUTE QUERY2;
DEALLOCATE PREPARE QUERY2;

#Insert into the "temporary" table
set @dsql=concat('
insert into ', @tablename, ' 
select e.EventUID, e.EventTimestamp, e.HasAudit, gn.GroupName, epi.UserID, eai.EventUID as `EventAuditUID`
    , concat(concat(concat(max(concat('' '', ui.UserPropertyValue)), '' (''), ut.UserName), '')'') as `ReviewerName`
from EventCore e
    inner join EventParticipantInformation epi on e.EventUID = epi.EventUID and epi.TypeClass=''FROM''
    inner join UserGroupRelation ugr on epi.UserID = ugr.UserID and e.EventTimestamp between ugr.EffectiveStartDate and ugr.EffectiveEndDate 
    inner join GroupNames gn on ugr.GroupID = gn.GroupID
    left outer join EventAuditInformation eai on e.EventUID = eai.EventUID
    left outer join UserTable ut on eai.UserID = ut.UserID
    left outer join UserInformation ui on eai.UserID = ui.UserID and ui.UserProperty=-10
    where e.EventTimestamp between @StartDate and @EndDate
        and e.SenderSID = @FirmID
    group by e.EventUID;');
PREPARE QUERY3 FROM @dsql;
EXECUTE QUERY3;
DEALLOCATE PREPARE QUERY3;

#Generate the actual query to return results. 
set @dsql=concat('
select rl1.GroupName as `Group`, coalesce(max(rl1.ReviewerName), '''') as `Reviewer(s)`, count(distinct rl1.EventUID) as `Total Events`
    , (count(distinct rl1.EventUID) - count(distinct rl1.EventAuditUID)) as `Unreviewed Events`
    , round(((count(distinct rl1.EventUID) - count(distinct rl1.EventAuditUID)) / count(distinct rl1.EventUID)) * 100, 1) as `% Unreviewed`
    , date_format(min(rl2.EventTimestamp), ''%W, %b %c %Y %r'') as `Oldest Unreviewed`
    , count(distinct rl3.EventUID) as `<=7 Days Unreviewed`
    , count(distinct rl4.EventUID) as `8-14 Days Unreviewed`
    , count(distinct rl5.EventUID) as `>14 Days Unreviewed`
from ', @tablename, ' rl1
left outer join ', @tablename, ' rl2 on rl1.EventUID = rl2.EventUID and rl2.EventAuditUID is null
left outer join ', @tablename, ' rl3 on rl1.EventUID = rl3.EventUID and rl3.EventAuditUID is null and rl1.EventTimestamp > DATE_SUB(NOW(), INTERVAL 7 DAY) 
left outer join ', @tablename, ' rl4 on rl1.EventUID = rl4.EventUID and rl4.EventAuditUID is null and rl1.EventTimestamp between DATE_SUB(NOW(), INTERVAL 7 DAY) and DATE_SUB(NOW(), INTERVAL 14 DAY)
left outer join ', @tablename, ' rl5 on rl1.EventUID = rl5.EventUID and rl5.EventAuditUID is null and rl1.EventTimestamp < DATE_SUB(NOW(), INTERVAL 14 DAY)
group by rl1.GroupName
order by ((count(distinct rl1.EventUID) - count(distinct rl1.EventAuditUID)) / count(distinct rl1.EventUID)) * 100 desc
;');
PREPARE QUERY4 FROM @dsql;
EXECUTE QUERY4;
DEALLOCATE PREPARE QUERY4;

#Drop "temporary" table
set @dsql = concat('drop table if exists ', @tablename, ';');
PREPARE QUERY5 FROM @dsql;
EXECUTE QUERY5;
DEALLOCATE PREPARE QUERY5;

Answer 4

Personally I'd just make it a permanent table. You might want to create a separate database for these tables (presumably they'll need unique names as lots of these queries could be done at once), also to allow permissions to be set sensibly (You can set permissions on databases; you can't set permissions on table wildcards).

Then you'd also need a cleanup job to remove old ones occasionally (MySQL conveniently remembers the time a table was created, so you could just use that to work out when a clean up was required)

Answer 5

I was able to change the query to a permanent table and this fixed it for me. ( changed the VLDB settings in MicroStrategy, temporary table type).

Answer 6

You can get around it by either making a permanent table, which you will remove afterwards, or just make 2 separate temp tables with the same data