Is there any harm in putting in a massive number into the len parameter of the Read() method?

You cannot read more than 3641144 characters from a stream to a string, as this is the limit for long strings: String Length Limit

From the documentation for the Read() method:

If no len is passed in, ie. 'Read()' then it is up to the Read implementation as to
how much data to return. Some stream classes use this to optimize the amount of
data returned to align this with the underlying storage of the stream.

So you can write like this:
Set inMsg %request.Content.Read($$$MaxLocalLength)

Macros are defined in %msql.inc:

#define MaxStringLength 3641144
#define MaxLocalLength $zutil(96,39)

there are a few comments:

  • you can write shorter:
    Property CarNumber As %String(TRUNCATE 1) [ SqlComputeCode = {{*}=$s(Clear:$$Decrypt({CAR_Number}),1:{CAR_Number})}, SqlComputedSqlFieldName CAR_Number ];
  • you can make a separate calculated field for the decrypted value, for example:
    Property CarNumber As %String(TRUNCATE 1) [ SqlFieldName CAR_Number ];
    
    Property CarNumberDecr As %String(TRUNCATE 1) [ CalculatedSqlComputeCode = {{*}=$s(Clear:$$Decrypt({CAR_Number}),1:{CAR_Number})}, SqlComputedSqlFieldName CAR_NumberDecr ];
  • where do you get the value of the Clear variable (see above)?
  • you can create your own data type for encrypted strings and override the corresponding methods there: How Data Type Classes Work

    In this case, you will not have to clutter up your queries with unnecessary encryption/decryption operations.

There are a couple of points:

  • what's wrong with users, roles, privileges, applications, etc.?
    %SYS>do ^SECURITY
     
     
    1) User setup
    2) Role setup
    3) Service setup
    4) Resource setup
    5) Application setup
    6) Auditing setup
    8) SSL configuration setup
    9) Mobile phone service provider setup
    10) OpenAM Identity Services setup
    11) Encryption key setup
    12) System parameter setup
    13) X509 User setup
    14) KMIP server setup
    15) Exit
     
    Option? 12
     
    1) Edit system options
    2) Edit authentication options
    3) Edit LDAP options
    4) Display system options
    5) Export All Security settings
    6) Import All Security settings
    7) Exit
     
    Option? 5
     
    Export ALL security records? Yes => Yes
     
    Warning: Before importing SSL configurations into a different configuration
    the same certificate directories and certificate files must exist in
    the new configuration, otherwise the import will fail.
     
    Export to file name SecurityExport.xml =>
    Parameters? "WNS" =>
    Confirm export of selected security records to SecurityExport.xml? No => yes
     
    Exported x Application security records
    Exported x DocDB security records
    Exported x Event security records
    Exported x KMIPServer security records
    Exported x LDAPConfig security records
    Exported x OpenAMIdentityService security records
    Exported x PhoneProvider security records
    Exported x Resource security records
    Exported x Role security records
    Exported x SQLPrivileges security records
    Exported x SSLConfig security records
    Exported x Service security records
    Exported x System security records
    Exported x User security records
    Exported x X509Credential security records
    Exported x X509Users security records
     
    Export complete
     
    1) Edit system options
    2) Edit authentication options
    3) Edit LDAP options
    4) Display system options
    5) Export All Security settings
    6) Import All Security settings
    7) Exit
     
    Option?
  • to transfer JDBC/ODBC settings, you can still use external tools, for example DbVisualizer or SQL Data Lens, where you can choose the data format (CSV, XML, JSON, Excel, etc.), the necessary fields and much more.

Did you copy/paste the query correctly here?

Select Books.nam,Books.print,Books.Relativename,Books.Firstpage,Books.Lastpage,Books.trn,Books.lastissue,
Books.firstissue,Books.person,Books.author,Books.price,Books.cd,Books.ab,Books.pf,Books.ju,Books.er,
Books.qw,Books.qt,Books.mn,
Records.qw,Records.er,Records.ty,Records.ui,Records.op,Records."as",
OrderBooks.mn,OrderBooks.bv,
OrderRecords.sd,OrderRecords.fg,OrderRecords.hj,
Orders.lastdate

From SQLUser.Books Books

INNER JOIN SQLUser.Records Records ON Books.id=Recordsid
INNER JOIN SQLUser.OrderedBooks OrderBooks ON Books.id=OrderBooks.id
INNER JOIN SQLUser.OrderedRecords OrderRecords ON Books.id=OrderRecords
INNER JOIN SQLUser.Orders Orders ON Books.id=Orders.id

where ((Records.qw=OrderBooks.qw) and (Records.er is null))

In addition to tuning tables, I would first look at the query plan and, accordingly, the indexes involved in it.

If you select the text with the mouse, you will see spaces on the right in the second "Output".
In general, I think that such details should be enclosed in quotation marks.