Function shouldUseQueryCacheForSubquery

src/Planner/Planner.cpp:2063–2091

Returns true if the Planner-level query result cache (with `is_subquery = true` key) should be used. For actual subqueries (is_subquery=true): explicit SETTINGS `use_query_cache` on the node wins. For all Planner invocations: `query_cache_for_subqueries` propagation from outer query. By default, `use_query_cache` does NOT propagate. Regardless of opt-in, never use the cache for execution kinds wh

/// per-replica or per-internal entries that the outer query already manages or that bypass the
/// safety gate `executeQuery` applies via `Context::setCanUseQueryResultCache`.
static bool shouldUseQueryCacheForSubquery(
    const QueryNode & query_node, bool outer_can_use_cache, const Settings & settings, bool is_subquery, const ContextPtr & query_context)
{
    /// Gate every Planner-level cache use by the query kind, so explicit `SETTINGS use_query_cache`
    /// on a subquery cannot bypass the safety check that `executeQuery` performs for the outer query.
    if (query_context->isInternalQuery()
        || query_context->getClientInfo().query_kind != ClientInfo::QueryKind::INITIAL_QUERY)
        return false;

    /// Only check explicit per-node `use_query_cache` for actual subqueries.
    /// For the top-level query, this setting is handled by `executeQuery` (with `is_subquery = false` key).
    if (is_subquery && query_node.hasSettingsChanges())
    {
        for (const auto & change : query_node.getSettingsChanges())
        {
            if (change.name == "use_query_cache")
                return change.value.safeGet<bool>();
        }
    }

    /// `query_cache_for_subqueries` enables the Planner-level (`is_subquery = true`) cache
    /// only for actual subqueries. The top-level query is cached separately by `executeQuery`
    /// using the `is_subquery = false` key; allowing it through this path here would write
    /// a duplicate entry under the subquery namespace.
    if (is_subquery && settings[Setting::query_cache_for_subqueries] && outer_can_use_cache)
        return true;

    return false;
}

void Planner::buildPlanForQueryNode()
{