Informix Error -502

Reasons / Common Causes

-502 fires while the database server is physically rebuilding a table in clustered order (as part of ALTER INDEX ... TO CLUSTER) and something goes wrong partway through that rebuild.

1. Insufficient free space in the dbspace holding the table, since clustering builds a complete second copy of the table before the original is dropped — per the official guidance, the typical cause.
2. An underlying ISAM-level error during the rebuild (a lock conflict, an I/O error, a constraint violation surfacing mid-copy) — the official text points to checking the accompanying ISAM error code for the specific reason.
3. Clustering attempted on a very large table without first confirming the target dbspace has headroom for a full second copy of the table plus its indexes.

Solutions / Resolution

1. Check the accompanying ISAM error reported alongside -502 — it identifies the specific underlying failure rather than leaving it to guesswork.
2. Check free space in the table's dbspace before retrying, since a shortage is the typical cause per the official guidance:

   onstat -d
   

   to see chunk-level free-page counts for the dbspace(s) involved.
3. Free space or add chunks to the dbspace, then retry the ALTER INDEX ... TO CLUSTER statement.

Examples

Diagnosing a clustering failure

ALTER INDEX idx_orders_new TO CLUSTER;
-- -502: Cannot cluster index.
-- (accompanying ISAM error, e.g. -111, reported alongside)

$ onstat -d
-- review free-page counts for the chunk(s) backing the table's dbspace

After freeing space

ALTER INDEX idx_orders_new TO CLUSTER;
-- succeeds once the dbspace has room for the rebuilt copy

Diagnostic Checks

1. Read the ISAM error code accompanying -502 first — it narrows the cause before any dbspace investigation is needed.
2. Run onstat -d to confirm free-page availability in the relevant dbspace when the accompanying error points toward a space shortage.

Related Errors / Related Topics

• -500 — "Clustered index index-name already exists in the table." A different clustering restriction: cardinality, not a rebuild failure.
• -501 — "Index index-name is already not clustered." The companion no-op warning on the un-clustering side.

The message names the operation (clustering) but not the reason; the accompanying ISAM error is where the actual cause lives.