### Description

![image](https://github.com/elastic/elasticsearch/assets/76245/d0443170-0419-4759-b505-7620ff3d6594)

The execution time of grouping stats by multiple keywords has increased: 1.2 to 1.6 seconds (with doc_partitioning), 4.3 to 6.7 seconds (with shard_partitioning). The equivalent query with the _search API takes 1.8 seconds.

`from nyc_taxis | ... | stats c = count(dropoff_datetime) by trip_type, payment_type, vendor_id`

This regression occurred since July 2, 2023, and I suspect it may be due to the PR: [Support multivalued fields on arbitrary grouping](https://github.com/elastic/elasticsearch-internal/pull/1340).

/cc @nik9000
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/BatchEncoder.java
/**
     * Checks if an offset is {@code null}.
     */
    public interface IsNull {
        boolean isNull(int offset);
    }
    /**
     * Decodes values encoded by {@link BatchEncoder}.
     */
    public interface Decoder {
        void decode(Block.Builder builder, IsNull isNull, BytesRef[] encoded, int count);
    }
    /**
     * Get a {@link Decoder} for the provided {@link ElementType}.
     */
    public static Decoder decoder(ElementType elementType) {
        return switch (elementType) {
            case INT -> new IntsDecoder();
            case LONG -> new LongsDecoder();