**Describe the situation**

Currently it does not:

```
milovidov-desktop :) EXPLAIN SYNTAX SELECT sum(cityHash64(*)) AS x FROM test.hits WHERE URLDomain LIKE '%metrika%'

EXPLAIN SYNTAX
SELECT sum(cityHash64(*)) AS x
FROM test.hits
WHERE URLDomain LIKE '%metrika%'

Query id: a752ed76-a71c-4e26-96c1-1b081abfc539

┌─explain────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ SELECT sum(cityHash64(WatchID, JavaEnable, Title, GoodEvent, EventTime, EventDate, CounterID, ClientIP, ClientIP6, RegionID, UserID, CounterClass, OS, UserAgent, URL, Referer, URLDomain, RefererDomain, Refresh, IsRobot, RefererCategories, URLCategories, URLRegions, RefererRegions, ResolutionWidth, ResolutionHeight, ResolutionDepth, FlashMajor, FlashMinor, FlashMinor2, NetMajor, NetMinor, UserAgentMajor, UserAgentMinor, CookieEnable, JavascriptEnable, IsMobile, MobilePhone, MobilePhoneModel, Params, IPNetworkID, TraficSourceID, SearchEngineID, SearchPhrase, AdvEngineID, IsArtifical, WindowClientWidth, WindowClientHeight, ClientTimeZone, ClientEventTime, SilverlightVersion1, SilverlightVersion2, SilverlightVersion3, SilverlightVersion4, PageCharset, CodeVersion, IsLink, IsDownload, IsNotBounce, FUniqID, HID, IsOldCounter, IsEvent, IsParameter, DontCountHits, WithHash, HitColor, UTCEventTime, Age, Sex, Income, Interests, Robotness, GeneralInterests, RemoteIP, RemoteIP6, WindowName, OpenerName, HistoryLength, BrowserLanguage, BrowserCountry, SocialNetwork, SocialAction, HTTPError, SendTiming, DNSTiming, ConnectTiming, ResponseStartTiming, ResponseEndTiming, FetchTiming, RedirectTiming, DOMInteractiveTiming, DOMContentLoadedTiming, DOMCompleteTiming, LoadEventStartTiming, LoadEventEndTiming, NSToDOMContentLoadedTiming, FirstPaintTiming, RedirectCount, SocialSourceNetworkID, SocialSourcePage, ParamPrice, ParamOrderID, ParamCurrency, ParamCurrencyID, GoalsReached, OpenstatServiceName, OpenstatCampaignID, OpenstatAdID, OpenstatSourceID, UTMSource, UTMMedium, UTMCampaign, UTMContent, UTMTerm, FromTag, HasGCLID, RefererHash, URLHash, CLID, YCLID, ShareService, ShareURL, ShareTitle, `ParsedParams.Key1`, `ParsedParams.Key2`, `ParsedParams.Key3`, `ParsedParams.Key4`, `ParsedParams.Key5`, `ParsedParams.ValueDouble`, IslandID, RequestNum, RequestTry)) AS x │
│ FROM test.hits                                                                                                                                                                                                                                             │
│ PREWHERE URLDomain LIKE '%metrika%'                                                                                                                                                                                                                        │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

3 rows in set. Elapsed: 0.004 sec. 

milovidov-desktop :) EXPLAIN SYNTAX SELECT sum(cityHash64(*)) AS x FROM merge(test, '^hits$') WHERE URLDomain LIKE '%metrika%'

EXPLAIN SYNTAX
SELECT sum(cityHash64(*)) AS x
FROM merge(test, '^hits$')
WHERE URLDomain LIKE '%metrika%'

Query id: fb059673-7da1-4cf4-9713-e5c7b9feb66d

┌─explain────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ SELECT sum(cityHash64(WatchID, JavaEnable, Title, GoodEvent, EventTime, EventDate, CounterID, ClientIP, ClientIP6, RegionID, UserID, CounterClass, OS, UserAgent, URL, Referer, URLDomain, RefererDomain, Refresh, IsRobot, RefererCategories, URLCategories, URLRegions, RefererRegions, ResolutionWidth, ResolutionHeight, ResolutionDepth, FlashMajor, FlashMinor, FlashMinor2, NetMajor, NetMinor, UserAgentMajor, UserAgentMinor, CookieEnable, JavascriptEnable, IsMobile, MobilePhone, MobilePhoneModel, Params, IPNetworkID, TraficSourceID, SearchEngineID, SearchPhrase, AdvEngineID, IsArtifical, WindowClientWidth, WindowClientHeight, ClientTimeZone, ClientEventTime, SilverlightVersion1, SilverlightVersion2, SilverlightVersion3, SilverlightVersion4, PageCharset, CodeVersion, IsLink, IsDownload, IsNotBounce, FUniqID, HID, IsOldCounter, IsEvent, IsParameter, DontCountHits, WithHash, HitColor, UTCEventTime, Age, Sex, Income, Interests, Robotness, GeneralInterests, RemoteIP, RemoteIP6, WindowName, OpenerName, HistoryLength, BrowserLanguage, BrowserCountry, SocialNetwork, SocialAction, HTTPError, SendTiming, DNSTiming, ConnectTiming, ResponseStartTiming, ResponseEndTiming, FetchTiming, RedirectTiming, DOMInteractiveTiming, DOMContentLoadedTiming, DOMCompleteTiming, LoadEventStartTiming, LoadEventEndTiming, NSToDOMContentLoadedTiming, FirstPaintTiming, RedirectCount, SocialSourceNetworkID, SocialSourcePage, ParamPrice, ParamOrderID, ParamCurrency, ParamCurrencyID, GoalsReached, OpenstatServiceName, OpenstatCampaignID, OpenstatAdID, OpenstatSourceID, UTMSource, UTMMedium, UTMCampaign, UTMContent, UTMTerm, FromTag, HasGCLID, RefererHash, URLHash, CLID, YCLID, ShareService, ShareURL, ShareTitle, `ParsedParams.Key1`, `ParsedParams.Key2`, `ParsedParams.Key3`, `ParsedParams.Key4`, `ParsedParams.Key5`, `ParsedParams.ValueDouble`, IslandID, RequestNum, RequestTry)) AS x │
│ FROM merge('test', '^hits$')                                                                                                                                                                                                                               │
│ WHERE URLDomain LIKE '%metrika%'                                                                                                                                                                                                                           │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```

Most likely because Merge table does not provide column sizes info.
We can provide it either:
- by iterating and summing across all tables;
- only for first N = 10 tables in case if the number of tables is very large.
{
    try
    {
        checkStackSize();
    }
    catch (Exception & e)
    {
        e.addMessage("while getting table iterator of Merge table. Maybe caused by two Merge tables that will endlessly try to read each other's data");
        throw;
    }
    DatabaseTablesIterators database_table_iterators;
    
    if (!database_is_regexp)
        database_table_iterators.emplace_back(getDatabaseIterator(source_database_name_or_regexp, local_context));
    
    else
    {
        auto databases = DatabaseCatalog::instance().getDatabases();
        for (const auto & db : databases)
        {
            if (source_database_regexp->match(db.first))
                database_table_iterators.emplace_back(getDatabaseIterator(db.first, local_context));
        }
    }
    return database_table_iterators;
}
void StorageMerge::checkAlterIsPossible(const AlterCommands & commands, ContextPtr local_context) const