presto-spi/src/main/java/io/prestosql/spi/block/MapHashTables.java/MapHashTables/buildAllHashTablesIfNecessary
class MapHashTables:    void buildAllHashTablesIfNecessary(Block rawKeyBlock, int[] offsets, @Nullable boolean[] mapIsNull)
    {
        // this is double checked locking
        if (hashTables == null) {
            buildAllHashTables(rawKeyBlock, offsets, mapIsNull);
        }
    }