MySQL Source Code Analysis: How a Record Is Determined to Match a WHERE Clause

1. Preparation

We create a test table t1 and insert sample rows, then run the example SQL:

SELECT * FROM t1
WHERE i2 > 20 AND (i1 = 50 OR i1 = 80);

2. Overall Introduction

In MySQL the WHERE clause is represented as a tree of Item objects. The tree for the example consists of an Item_cond_and node that connects an Item_func_gt (i2 > 20) and an Item_cond_or (i1 = 50 OR i1 = 80). Each leaf is an Item_func_eq or Item_func_gt that holds an Item_field (the column) and an Item_int (the constant).

3. Source Code Analysis

3.1 ExecuteIteratorQuery()

bool Query_expression::ExecuteIteratorQuery(THD *thd) {
  ...
  for (;;) {
    int error = m_root_iterator->Read();
    if (error > 0 || thd->is_error())
      return true;               // fatal error
    else if (error < 0)
      break;                     // no more rows
    else if (thd->killed) {
      thd->send_kill_message();
      return true;               // aborted by user
    }
    if (query_result->send_data(thd, *fields))
      return true;
    ...
  }
  ...
}

This is the entry point for a SELECT statement. For our example the root iterator is a FilterIterator , which reads rows from the storage engine and then evaluates the WHERE condition.

3.2 FilterIterator::Read()

int FilterIterator::Read() {
  for (;;) {
    int err = m_source->Read();
    if (err != 0) return err;
    bool matched = m_condition->val_int();
    if (thd()->killed) {
      thd()->send_kill_message();
      return 1;
    }
    if (thd()->is_error()) return 1;
    if (!matched) {
      m_source->UnlockRow();
      continue;
    }
    return 0;   // successful row
  }
}

The iterator reads a row, then calls Item_cond_and::val_int() (the root condition) to decide whether the row matches.

3.3 compare_int_signed()

int Arg_comparator::compare_int_signed() {
  longlong val1 = (*left)->val_int();
  if (current_thd->is_error()) return 0;
  if (!(*left)->null_value) {
    longlong val2 = (*right)->val_int();
    if (current_thd->is_error()) return 0;
    if (!(*right)->null_value) {
      if (set_null) owner->null_value = false;
      if (val1 < val2) return -1;
      if (val1 == val2) return 0;
      return 1;
    }
  }
  if (set_null) owner->null_value = true;
  return -1;
}

This function compares the left‑hand side and right‑hand side integer values of a comparison operator, handling NULLs and returning -1, 0, or 1.

3.4 Arg_comparator::compare()

inline int Arg_comparator::compare() { return (this->*func)(); }

The func pointer is set to the appropriate comparison routine (e.g., compare_int_signed ) during query preparation.

3.5 Item_func_gt::val_int()

longlong Item_func_gt::val_int() {
  assert(fixed == 1);
  int value = cmp.compare();
  return value > 0 ? 1 : 0;
}

Evaluates the ‘greater‑than’ condition by delegating to the comparator and returns 1 if the left value is larger.

3.6 Item_cond_and::val_int()

longlong Item_cond_and::val_int() {
  List_iterator_fast
li(list);
  Item *item;
  null_value = false;
  while ((item = li++)) {
    if (!item->val_bool()) {
      if (ignore_unknown() || !(null_value = item->null_value))
        return 0;   // false
    }
    if (current_thd->is_error()) return error_int();
  }
  return null_value ? 0 : 1;
}

The method iterates over all AND‑connected conditions; all must evaluate to true. If any condition is false and either ignore_unknown() is true or the condition is not NULL, the whole expression returns false. NULL handling is controlled by ignore_unknown() .

3.7 Item_func_eq::val_int()

longlong Item_func_eq::val_int() {
  assert(fixed == 1);
  int value = cmp.compare();
  return value == 0 ? 1 : 0;
}

Returns 1 when the left and right operands are equal.

3.8 Item_cond_or::val_int()

longlong Item_cond_or::val_int() {
  List_iterator_fast
li(list);
  Item *item;
  null_value = false;
  while ((item = li++)) {
    if (item->val_bool()) {
      null_value = false;
      return 1;   // true
    }
    if (item->null_value) null_value = true;
  }
  return 0;       // false
}

The OR node returns true as soon as any sub‑condition evaluates to true; otherwise it returns false, propagating NULL information when needed.

4. Summary

The article demonstrates how MySQL evaluates WHERE clauses that contain AND and OR operators. After a row is fetched, Item_cond_and::val_int() checks that every AND sub‑condition is true, while Item_cond_or::val_int() succeeds if any OR sub‑condition is true. The comparison logic is performed by compare_int_signed() and the surrounding wrapper classes, with careful handling of NULL values via ignore_unknown() and the null_value flag.