Q61485: Maximum Number of ISAM Files Open at Once in BASIC 7.00/7.10

Article: Q61485
Product(s): See article
Version(s): 7.00 7.10 | 7.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S900408-3 | mspl13_basic
Last Modified: 6-FEB-1991

The number of ISAM tables that you can have open at one time is
determined by the number of database files that you have open. (See
table below.)

The maximum number of database files that you can have open at once is
4 files. However, this is not the maximum number of tables you can
have open at once. The maximum number of tables that you can have open
at once is 13 tables in 1 database file, 10 tables in 2 database
files, 7 tables in 3 database files, or 4 tables in 4 database files.

Attempting to open more than the maximum allowed number of tables or
database files at once gives error 67, "Too many files."

A database file in Microsoft BASIC Professional Development System
(PDS) version 7.00 or 7.10 is created with the default extension .MDB
and contains the physical data for each table plus the indexes used to
define and point to the data.

This information applies to Microsoft BASIC Professional Development
System (PDS) versions 7.00 and 7.10 for MS-DOS and 7.10 for MS OS/2.

The ISAM engine in Microsoft BASIC PDS 7.00 and 7.10 has a maximum
number of 16 pointers or ISAM handles that it can use with ISAM files.
These are NOT the same as DOS file handles.

Each file still takes one DOS file handle for its first open, but it
doesn't need any additional DOS file handles for additional tables
opened in the same database file.

Each database file takes four ISAM handles on its initial open: one
handle for the file, one for the indexes, one for the data, and one
for the initial table. Each additional table referenced in an already
opened database takes only one additional ISAM handle and no more DOS
handles. This is because ISAM handles that point to the same file,
indexes, and data already exist.

As a result, the number of tables that you can have open at once is
determined by the number of database files you are using. The
following table gives the combinations of databases (.MDBs) and
additional ISAM tables that you can have open at one time:

            Maximum Number of ISAM Files and Tables

   ---------------------------------------------------------
   # of .MDBs (files):  |    1         2        3        4
                        |
   # of additional      |
   tables in already    |    12        8        4        0
   opened .MDBs:        |
                        |
   Total tables:        |    13        10       7        4
                        |
                        |
   Total ISAM handles:  |    16        16       16       16
   ---------------------------------------------------------

Therefore, if you have all of your tables in only 1 database file, you
can open a total of 13 tables at once. However, if you open tables in
4 database files, you can open only those 4 tables at once.

This information is taken from page 388 of the "Microsoft BASIC 7.0:
Programmer's Guide" for versions 7.00 and 7.10 (Chapter 10, "Database
Programming with ISAM").

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