Informix Error -158: ISAM error: Operation disallowed on SMI pseudo table.

Reasons / Common Causes

-158 means an operation was attempted against a System Monitoring Interface (SMI) pseudo table — one of the sysmaster database's virtual tables — that isn't supported there. These tables aren't genuine physical structures; they're generated on the fly from live engine internals, so they don't support the full range of operations an ordinary table does.

1. A write operation (INSERT/UPDATE/DELETE) attempted against an SMI pseudo-table — these are read-only views into live engine state, not real, writable data.
2. DDL attempted against an SMI table (CREATE INDEX, ALTER TABLE, and similar) — not supported, since these aren't physical structures the engine manages that way.
3. A query construct unsupported specifically against SMI tables, even for a plain SELECT — some SMI tables restrict certain joins, functions, or locking hints because of how they're generated from internal engine structures rather than read from disk normally.
4. Generic tooling or an ORM treating sysmaster tables as ordinary tables, assuming uniform capabilities across every table in a database without special-casing the sysmaster database's virtual nature.
5. Monitoring or reporting tooling attempting an operation that works on ordinary tables but isn't supported here — trying to add an index for query performance, for example, on a table that isn't stored the way indexing assumes.

Solutions / Resolution

1. Restrict operations against SMI pseudo-tables to supported read (SELECT) operations — per the official guidance, try a different, supported operation instead of the one that failed.
2. Treat sysmaster tables as read-only telemetry sources for monitoring or reporting needs — never attempt writes or DDL against them.
3. Review generic tooling or ORM configuration to exclude or special-case the sysmaster database, since its tables don't behave like ordinary user tables and shouldn't be treated uniformly by tooling that assumes they do.
4. If a query against an SMI table needs functionality that isn't supported directly (an index for performance, for example), select the needed data into a real temporary table first, then perform the additional operation against that copy instead of the SMI table directly.
5. Consult the documentation for the specific SMI table involved, since supported operations can vary somewhat from one SMI table to another.

Examples

Attempting to write to a monitoring table

INSERT INTO sysmaster:syssesprof VALUES (...);
-- -158: this is a read-only view into live session profiling data,
-- not a table you can write to

Working around an unsupported operation via a temp table

-- Instead of attempting an unsupported operation directly on the SMI table:
SELECT * INTO TEMP tmp_sessions FROM sysmaster:syssessions;
-- now perform whatever additional operation is needed against
-- tmp_sessions, a genuine, ordinary temporary table

Generic tooling stumbling on sysmaster

A schema-introspection tool that iterates over every table in every database and attempts a uniform operation (adding a monitoring index, for example) hits -158 the moment it reaches sysmaster's pseudo-tables — the fix is excluding sysmaster from that tool's generic handling, not finding a way to make the operation work there.

Diagnostic Checks

1. Confirm the target table is genuinely an SMI/sysmaster pseudo-table rather than an ordinary table that happens to have a similar name.
2. Review exactly which operation was attempted — a write, DDL, or an unsupported SELECT variant — against the table.
3. Check documentation for the specific SMI table involved, since the exact set of supported operations isn't identical across every SMI table.

Related Errors / Related Topics

• -100 — "ISAM error: duplicate value for a record with unique key." The other foundational ISAM-level error in this family, though unrelated in cause — grouped here as background on how this error class is organized.

If a monitoring or reporting workflow needs to do more than read from an SMI table, copy the needed data into a real temporary table first rather than trying to make the unsupported operation work directly against the pseudo-table.