# mysqlnd_qc_set_is_select

(PECL mysqlnd_qc >= 1.0.0)

mysqlnd_qc_set_is_select — Installs a callback which decides whether a statement is cached

### 说明

mysqlnd_qc_set_is_select ( string `$callback` ) : mixed

Installs a callback which decides whether a statement is cached.

There are several ways of hinting PELC/mysqlnd_qc to cache a query. By default, PECL/mysqlnd_qc attempts to cache a if caching of all statements is enabled or the query string begins with a certain SQL hint. The plugin internally calls a function named is_select() to find out. This internal function can be replaced with a user-defined callback. Then, the user-defined callback is responsible to decide whether the plugin attempts to cache a statement. Because the internal function is replaced with the callback, the callback gains full control. The callback is free to ignore the configuration setting mysqlnd_qc.cache_by_default and SQL hints.

The callback is invoked for every statement inspected by the plugin. It is given the statements string as a parameter. The callback returns `FALSE` if the statement shall not be cached. It returns `TRUE` to make the plugin attempt to cache the statements result set, if any. A so-created cache entry is given the default TTL set with the PHP configuration directive mysqlnd_qc.ttl. If a different TTL shall be used, the callback returns a numeric value to be used as the TTL.

The internal is_select function is part of the internal cache storage handler interface. Thus, a user-defined storage handler offers the same capabilities.

### 参数

此函数没有参数。

### 返回值

成功时返回 `TRUE`， 或者在失败时返回 `FALSE`。

### 范例

Example #1 mysqlnd_qc_set_is_select() example

    <?php
    /* callback which decides if query is cached */
    function is_select($query) {
      static $patterns = array(
       /* true - use default from mysqlnd_qc.ttl */
       "@SELECT\s+.*\s+FROM\s+test@ismU" => true,
       /* 3 - use TTL = 3 seconds */
       "@SELECT\s+.*\s+FROM\s+news@ismU" => 3
      );
      /* check if query does match pattern */
      foreach ($patterns as $pattern => $ttl) {
        if (preg_match($pattern, $query)) {
          printf("is_select(%45s): cache\n", $query);
          return $ttl;
        }
      }
      printf("is_select(%45s): do not cache\n", $query);
      return false;
    }
    mysqlnd_qc_set_is_select("is_select");

    /* Connect, create and populate test table */
    $mysqli = new mysqli("host", "user", "password", "schema");
    $mysqli->query("DROP TABLE IF EXISTS test");
    $mysqli->query("CREATE TABLE test(id INT)");
    $mysqli->query("INSERT INTO test(id) VALUES (1), (2), (3)");

    /* cache put */
    $mysqli->query("SELECT id FROM test WHERE id = 1");
    /* cache hit */
    $mysqli->query("SELECT id FROM test WHERE id = 1");
    /* cache put */
    $mysqli->query("SELECT * FROM test");
    ?>

以上例程会输出：

    is_select(                    DROP TABLE IF EXISTS test): do not cache
    is_select(                    CREATE TABLE test(id INT)): do not cache
    is_select(    INSERT INTO test(id) VALUES (1), (2), (3)): do not cache
    is_select(             SELECT id FROM test WHERE id = 1): cache
    is_select(             SELECT id FROM test WHERE id = 1): cache
    is_select(                           SELECT * FROM test): cache

### 参见

- Runtime configuration
- mysqlnd_qc.ttl
- mysqlnd_qc.cache_by_default
- mysqlnd_qc_set_user_handlers() - Sets the callback functions for a user-defined procedural storage handler