**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.
class ASTInsertQuery;
struct Settings;
class AlterCommands;
class MutationCommands;
struct PartitionCommand;
using PartitionCommands = std::vector<PartitionCommand>;
class IProcessor;
using ProcessorPtr = std::shared_ptr<IProcessor>;
using Processors = std::vector<ProcessorPtr>;
class Pipe;
class QueryPlan;
using QueryPlanPtr = std::unique_ptr<QueryPlan>;
class SinkToStorage;
using SinkToStoragePtr = std::shared_ptr<SinkToStorage>;
class QueryPipelineBuilder;
using QueryPipelineBuilderPtr = std::unique_ptr<QueryPipelineBuilder>;
class IStoragePolicy;
using StoragePolicyPtr = std::shared_ptr<const IStoragePolicy>;
struct StreamLocalLimits;
class EnabledQuota;