Method transform

src/Processors/Transforms/AddingDefaultsTransform.cpp:154–329

void AddingDefaultsTransform::transform(Chunk & chunk)
{
    if (column_defaults.empty())
        return;

    const auto * block_missing_values = input_format.getMissingValues();
    if (!block_missing_values)
        return;

    const auto & header = getOutputPort().getHeader();
    size_t num_rows = chunk.getNumRows();
    auto res = header.cloneWithColumns(chunk.detachColumns());

    /// Identify columns that need defaults computed
    std::vector<std::pair<String, size_t>> columns_needing_defaults;
    for (const auto & [col_name, col_default] : column_defaults)
    {
        if (!res.has(col_name))
            continue;
        size_t column_idx = res.getPositionByName(col_name);
        if (block_missing_values->hasDefaultBits(column_idx))
            columns_needing_defaults.emplace_back(col_name, column_idx);
    }

    if (columns_needing_defaults.empty())
    {
        chunk.setColumns(res.getColumns(), num_rows);
        return;
    }

    /// Build dependency graph: for each column needing defaults, find which other
    /// columns (that also need defaults) its default expression depends on.
    /// This is needed because if column `s` has DEFAULT concat('test', CAST(n, 'String'))
    /// and column `n` has DEFAULT 42, when inserting {"n": 2} we need to first compute
    /// n's defaults (for rows where n is missing), mix them into the block, and only
    /// then compute s's defaults so that s can see the correct values of n.
    std::unordered_map<String, NameSet> dependencies;
    std::unordered_set<String> columns_needing_defaults_set;
    for (const auto & [col_name, col_idx] : columns_needing_defaults)
        columns_needing_defaults_set.insert(col_name);

    for (const auto & [col_name, col_idx] : columns_needing_defaults)
    {
        const auto & col_default = column_defaults.at(col_name);
        RequiredSourceColumnsVisitor::Data columns_context;
        auto expr_clone = col_default.expression->clone();
        RequiredSourceColumnsVisitor(columns_context).visit(expr_clone);
        NameSet required = columns_context.requiredColumns();

        NameSet deps;
        for (const auto & req : required)
        {
            if (columns_needing_defaults_set.contains(req))
                deps.insert(req);
        }
        dependencies[col_name] = std::move(deps);
    }