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Summary
INFORMATION_SCHEMA was endorsed by the ISO/ANSI body longafter the real RDBMS implementations moved into the market. As a result, some
of the vendors implemented their own version of the metadata repository in a
form of system tables. The information from these tables can be
gathered in a variety of ways, usually through views, provided by the vendor
for just this purpose (the idea behind the
INFORMATION_SCHEMA), through some
vendor-supplied stored procedures or functions, or by making your best guess
via direct querying of the underlying system tables. Most vendors explicitly discourage users
from accessing the system tables directly because their structure might change
without any notice, and the information contained in the table is not
guaranteed to mean what you think it should; in short, system tables are for
the use of the system, views are for the users.
While introducing the standard is a step
in the right direction toward uniform interface to metadata, it probably will
not happen overnight (considering that
INFORMATION_SCHEMA was first introduced in
the SQL92 standard). The system catalog of the structure specified in the
SQL92/99 standard is a requirement for achieving higher levels of the standard
conformance; most of the RDBMS products are only Level 1 compliant and are
likely to remain so in the nearest future.
