What causes "Cannot unnest type: " in prestodb/presto?

Check the query: only UNNEST array/map/row-typed columns; wrap scalars in an array first, e.g. UNNEST(ARRAY[col]).

prestodb/presto · error · IllegalArgumentException

Cannot unnest type:

Error message

Cannot unnest type:

What it means

UNNEST expands arrays, maps, and rows into separate columns/rows. During operator setup, createUnnester maps each nested column type to an Unnester implementation; only ArrayType, MapType, and RowType are supported. Any other type reaching this switch is a planner bug or misuse and throws IllegalArgumentException.

Source

Thrown at presto-main-base/src/main/java/com/facebook/presto/operator/unnest/UnnestOperator.java:226

    }

    private static Unnester createUnnester(Type nestedType, boolean isLegacyUnnest)
    {
        if (nestedType instanceof ArrayType) {
            Type elementType = ((ArrayType) nestedType).getElementType();

            if (!isLegacyUnnest && elementType instanceof RowType) {
                return new ArrayOfRowsUnnester(elementType.getTypeParameters().size());
            }
            else {
                return new ArrayUnnester();
            }
        }
        else if (nestedType instanceof MapType) {
            return new MapUnnester();
        }
        else {
            throw new IllegalArgumentException("Cannot unnest type: " + nestedType);
        }
    }

    private void resetBlockBuilders()
    {
        for (int i = 0; i < replicateTypes.size(); i++) {
            Block newInputBlock = currentPage.getBlock(replicateChannels.get(i));
            replicatedBlockBuilders.get(i).resetInputBlock(newInputBlock);
        }

        int positionCount = currentPage.getPositionCount();
        maxLengths = ensureCapacity(maxLengths, positionCount, SMALL, INITIALIZE);

        for (int i = 0; i < unnestTypes.size(); i++) {
            int inputChannel = unnestChannels.get(i);
            Block unnestChannelInputBlock = currentPage.getBlock(inputChannel);
            Unnester unnester = unnesters.get(i);

View on GitHub (pinned to 55bb57d202)

Solutions

Check the query: only UNNEST array/map/row-typed columns; wrap scalars in an array first, e.g. UNNEST(ARRAY[col]).
If triggered from custom operator construction, verify nestedType instanceof ArrayType || MapType || RowType before calling createUnnester.
If reproducible with a plain query against stock types, report it as a planner bug with the query and EXPLAIN plan.

Example fix

// before
UnnestOperator with nestedType = VARCHAR (scalar)
// after
Wrap scalar in an array in SQL: SELECT ... FROM t CROSS JOIN UNNEST(ARRAY[scalar_col]) AS u(x)

Defensive patterns

Strategy: try-catch

Validate before calling

-- only unnest supported types in SQL; wrap scalars:
SELECT ... FROM t CROSS JOIN UNNEST(ARRAY[scalar_col]) AS u(x)

Try / catch

try {
    runUnnestQuery();
} catch (IllegalArgumentException e) {
    if (e.getMessage().startsWith("Cannot unnest type:")) {
        throw new QueryValidationError("Column passed to UNNEST must be array, map, or row type");
    }
    throw e;
}

Prevention

Only UNNEST array, map, or row typed expressions.
Inspect column types with DESCRIBE before unnesting dynamic inputs.
If building plans programmatically, assert types before constructing UnnestOperator.

When it happens

Trigger: An UNNEST query where the replicated/unnested input column type is not an array, map, or row — typically caused by a type change in the plan that the planner failed to catch, or by custom code constructing an UnnestOperator directly with an unsupported type.

Common situations: Custom connectors/plugins feeding unexpected types into unnest; internal bugs after type-system changes; passing a scalar (e.g. VARCHAR) column to UNNEST through a plugin-generated plan.

Related errors

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/4cf31b3a3ce31bab. Report an issue: GitHub.