Informix Error -872

Reasons / Common Causes

-872 fires in two related situations, per the official guidance: a unique index fragmented by round-robin (not supported for unique indexes at all), or a unique index fragmented by expression where the expression references a column not part of the index's own key.

1. Round-robin fragmentation attempted on a unique index, per the official guidance — this method simply isn't supported for unique indexes, the same underlying restriction referenced by -774's own example.
2. Expression-based fragmentation on a unique index, where the expression references a column outside the index's key columns, per the official guidance — every column used in the fragmentation expression must also be part of the index key.

Solutions / Resolution

1. Use a different fragmentation method for a unique index, per the official guidance, if round-robin was attempted — expression-based fragmentation (meeting the key-column requirement) or same-as-table fragmentation are the alternatives.
2. Ensure every column in the fragmentation expression is also part of the index's key columns, per the official guidance, if using expression-based fragmentation on a unique index.

Examples

Round-robin on a unique index

CREATE UNIQUE INDEX idx_orders_id ON orders (order_id)
  FRAGMENT BY ROUND ROBIN IN dbspace1, dbspace2;
-- -872: round-robin isn't supported for unique indexes

An expression referencing a non-key column

CREATE UNIQUE INDEX idx_orders_id ON orders (order_id)
  FRAGMENT BY EXPRESSION
  (region = 'east') IN dbspace1, REMAINDER IN dbspace2;
-- -872: 'region' isn't part of the index key (order_id)

Corrected — expression using only key columns

CREATE UNIQUE INDEX idx_orders_id ON orders (order_id)
  FRAGMENT BY EXPRESSION
  (order_id < 1000) IN dbspace1, REMAINDER IN dbspace2;

Diagnostic Checks

1. Check the fragmentation method — round-robin is never valid for a unique index.
2. For expression-based fragmentation, compare every column referenced in the expression against the index's key columns.

Related Errors / Related Topics

• -774 — "Cannot specify fragment expressions with a round-robin fragmentation." A related restriction — this error's round-robin case is the same underlying incompatibility, specific to unique indexes.
• -873 — "Invalid fragment expression column." A related restriction, about referencing columns from a different table entirely, rather than a column outside the index's own key.

Round-robin never works for unique indexes; for expression-based fragmentation, every referenced column must also be part of the index's key.