apache/cassandra · error · InvalidRequestException

Invalid element selection

Error message

Invalid element selection: %s is of type %s is not a collection

What it means

Thrown when preparing an element selection (collection[key] or collection[idx] in a SELECT) if the type of the selected column, after unwrapping ReversedType, is not a CollectionType. Element access is only defined for lists, sets and maps, so non-collection columns cannot be indexed with [].

Solutions

1. Verify the column type with DESC[RIBE] TABLE or system_schema.columns; ensure it is list/set/map
2. Remove the [element] accessor and select the whole column instead
3. If a reversed type was intended to wrap a collection, ensure the underlying base type is actually a collection

Example fix

// before
SELECT plain_text_col['key'] FROM ks.tbl;
// after
SELECT map_col['key'] FROM ks.tbl;  -- map_col must be a map/list/set

// verify column is a collection before element access
String type = getColumnType(keyspace, table, column); // from system_schema.columns
if (!type.matches("(list|set|map).*")) throw new IllegalArgumentException(column + " is not a collection; cannot use [element]");

try { session.execute(String.format("SELECT %s[?] FROM %s", col, table), key); } catch (InvalidRequestException e) { if (e.getMessage().contains("is not a collection")) { /* select whole column instead */ } else throw e; }

When it happens

Trigger: SELECT mycol[k] or mycol[idx] where mycol is not a list/set/map (e.g. a text, int, frozen non-collection, or a reversed wrapper around a non-collection).

Common situations: Copy-pasting element-access syntax onto a scalar column; assuming a frozen column is still a collection for element selection; schema changes that changed a column's type after queries were written.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• appendAll() can only be called on non-frozen collections
• Invalid slice selection
• Cannot create index on
• Cannot create () index on frozen column . Frozen…
• Cannot create () index on . Non-collection columns only…

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/c194161e34811035. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/selection/Selectable.java:1479

        @Override
        public String toString()
        {
            return String.format("%s[%s]", selected, element);
        }

        public Selector.Factory newSelectorFactory(TableMetadata cfm, AbstractType<?> expectedType, List<ColumnMetadata> defs, VariableSpecifications boundNames)
        {
            Selector.Factory factory = selected.newSelectorFactory(cfm, null, defs, boundNames);
            ColumnSpecification receiver = factory.getColumnSpecification(cfm);

            AbstractType<?> type = receiver.type;
            if (receiver.isReversedType())
            {
                type = ((ReversedType<?>) type).baseType;
            }
            if (!(type instanceof CollectionType))
                throw new InvalidRequestException(String.format("Invalid element selection: %s is of type %s is not a collection", selected, type.asCQL3Type()));

            ColumnSpecification boundSpec = specForElementOrSlice(selected, receiver, ((CollectionType) type).kind, "Element");

            Term elt = element.prepare(cfm.keyspace, boundSpec);
            elt.collectMarkerSpecification(boundNames, cfm);
            return ElementsSelector.newElementFactory(toString(), factory, (CollectionType)type, elt);
        }

        public AbstractType<?> getExactTypeIfKnown(String keyspace)
        {
            AbstractType<?> selectedType = selected.getExactTypeIfKnown(keyspace);
            if (selectedType == null || !(selectedType instanceof CollectionType))
                return null;

            return ElementsSelector.valueType((CollectionType) selectedType);
        }

        @Override

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