Bug Fix: Support array(varchar) results returned as JSON strings

[siddhuoo7]

When querying a Presto table with an array(varchar) column (e.g. from an Iceberg catalog), queries fail with:

cannot convert [ "CustomAnalytics Solutions", "PredictiveForecasting Module" ] (string) to slice

This happens because the Presto Go client’s typeConverter assumes that array(...) results are always []interface{}, but in practice the Presto REST API sometimes serializes array values as JSON-encoded strings. The SDK does not handle this case, resulting in runtime errors when scanning results.

Repro:

CREATE TABLE my_table (
  id bigint,
  products array(varchar)
);

INSERT INTO my_table (id, products)
VALUES (1, ARRAY['CustomAnalytics Solutions', 'PredictiveForecasting Module']);

Go code using the SDK:

rows, _ := conn.QueryContext(ctx, "SELECT id, products FROM my_table")

Result → ❌ cannot convert [...] (string) to slice

---

✅ Fix:

• Updated the "array" case in typeConverter.ConvertValue to support both []interface{} and string (JSON-encoded) inputs.
• If v is a string, attempt to json.Unmarshal it into a []interface{}.
• If v is already a []interface{}, return as-is.
• Preserves compatibility with existing behavior while handling additional response formats.

---

🔨 Implementation details:

case "array":
    switch val := v.(type) {
    case nil:
        return nil, nil
    case string:
        var arr []interface{}
        if err := json.Unmarshal([]byte(val), &arr); err != nil {
            return nil, fmt.Errorf("cannot parse array from string %q: %w", val, err)
        }
        return arr, nil
    case []interface{}:
        return val, nil
    default:
        return nil, fmt.Errorf("cannot convert %v (%T) to slice", v, v)
    }

---

🧪 Tests:

• Verified with array(varchar) column (Iceberg catalog).
• Verified with nested ARRAY[ARRAY['a','b']].
• Verified backward compatibility when Presto returns []interface{} instead of JSON strings.

---

[sourcery-ai]

Reviewer's Guide

Enhanced array handling in the Presto Go client by extending ConvertValue to parse JSON-encoded string arrays, refactoring slice normalization into a new helper, updating NullSliceString.Scan accordingly, bumping a dependency, and adding a CI workflow for integration tests.

Class diagram for updated array handling in Presto Go client

classDiagram
    class typeConverter {
        +ConvertValue(v interface{}) (driver.Value, error)
    }
    class NullSliceString {
        +SliceString []sql.NullString
        +Valid bool
        +Scan(value interface{}) error
    }
    class "normalizeToInterfaceSlice" {
        +normalizeToInterfaceSlice(value interface{}) ([]interface{}, error)
    }
    typeConverter --> "normalizeToInterfaceSlice": uses
    NullSliceString --> "normalizeToInterfaceSlice": uses
    NullSliceString --> sql.NullString: contains

Loading

Class diagram for new normalizeToInterfaceSlice helper

classDiagram
    class "normalizeToInterfaceSlice" {
        +normalizeToInterfaceSlice(value interface{}) ([]interface{}, error)
    }
    "normalizeToInterfaceSlice" <.. NullSliceString: used by
    "normalizeToInterfaceSlice" <.. typeConverter: used by

Loading

Flow diagram for array(varchar) value normalization

flowchart TD
    A["Presto REST API returns array(varchar) value"] --> B["typeConverter.ConvertValue receives value"]
    B --> C{Is value a string?}
    C -- Yes --> D["json.Unmarshal to []interface{}"]
    C -- No --> E{Is value []interface{}?}
    E -- Yes --> F["Return as-is"]
    E -- No --> G["Error: cannot convert to slice"]
    D --> H["Return []interface{}"]
    F --> H
    G --> I["Return error"]

Loading

File-Level Changes

Change	Details	Files
Support JSON-encoded string arrays in typeConverter.ConvertValue	Handle string inputs by unmarshaling JSON into []interface{} Support existing []interface{} inputs unchanged Return nil for nil values and error for other types	presto/presto.go
Remove obsolete slice validation	Deleted validateSlice function Removed its invocation in ConvertValue	presto/presto.go
Introduce normalizeToInterfaceSlice helper	Add new helper to convert various slice shapes (JSON strings, []byte, concrete slices) to []interface{} Use reflection to handle concrete slice types	presto/null_slices_helpers.go
Refactor NullSliceString.Scan to use normalization helper	Use normalizeToInterfaceSlice instead of manual type assertion Preserve nil-as-empty behavior and update error formatting Keep element scanning via existing scanNullString	presto/presto.go
Bump golang.org/x/crypto dependency	Update version to v0.31.0 in go.mod	go.mod
Add GitHub Actions workflow for integration tests	Create run-presto-tests.yml to set up Go, Docker and run integration tests	.github/workflows/run-presto-tests.yml

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