Always declare settings before using them and optimize access patterns for performance. Settings must be properly declared in the appropriate scope before being referenced in code. When accessing settings multiple times, cache the value in a local variable to avoid repeated lookups. For configuration parsing, avoid modifying AST nodes directly and use local variables instead.

// Declare setting before use
DECLARE(UInt64, database_replicated_logs_to_keep, 1000, "Description", 0)

// Cache setting value when used multiple times
bool enable_optimization = planner_context->getQueryContext()->getSettingsRef()[Setting::enable_add_distinct_to_in_subqueries];
for (const auto & item : items) {
    if (enable_optimization) {
        // Use cached value instead of repeated setting access
    }
}

// Avoid modifying AST, use local variables
auto key_value = evaluateConstantExpressionOrIdentifierAsLiteral(children[0], context);
// Store in local variable instead of updating children[0] directly

This practice prevents runtime errors from undeclared settings, improves performance by reducing repeated setting lookups, and maintains cleaner code by avoiding direct AST modifications during configuration parsing.

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