&reftitle.constants; &extension.constants;

SQL hint related

## Using SQL hint constants

The query cache is controlled by SQL hints. SQL hints are used to enable and disable caching. SQL hints can be used to set the `TTL` of a query.

The SQL hints recognized by the query cache can be manually changed at compile time. This makes it possible to use `mysqlnd_qc` in environments in which the default SQL hints are already taken and interpreted by other systems. Therefore it is recommended to use the SQL hint string constants instead of manually adding the default SQL hints to the query string.

    <?php
    /* Use constants for maximum portability */
    $query = "/*" . MYSQLND_QC_ENABLE_SWITCH . "*/SELECT id FROM test";

    /* Valid but less portable: default TTL */
    $query = "/*qc=on*/SELECT id FROM test";

    /* Valid but less portable: per statement TTL */
    $query = "/*qc=on*//*qc_ttl=5*/SELECT id FROM test";

    printf("MYSQLND_QC_ENABLE_SWITCH: %s\n", MYSQLND_QC_ENABLE_SWITCH);
    printf("MYSQLND_QC_DISABLE_SWITCH: %s\n", MYSQLND_QC_DISABLE_SWITCH);
    printf("MYSQLND_QC_TTL_SWITCH: %s\n", MYSQLND_QC_TTL_SWITCH);
    ?>

&examples.outputs;

    MYSQLND_QC_ENABLE_SWITCH: qc=on
    MYSQLND_QC_DISABLE_SWITCH: qc=off
    MYSQLND_QC_TTL_SWITCH: qc_ttl=

`MYSQLND_QC_ENABLE_SWITCH` (`string`)

- SQL hint used to enable caching of a query.

`MYSQLND_QC_DISABLE_SWITCH` (`string`)

- SQL hint used to disable caching of a query if `mysqlnd_qc.cache_by_default = 1`.

`MYSQLND_QC_TTL_SWITCH` (`string`)

- SQL hint used to set the TTL of a result set.

`MYSQLND_QC_SERVER_ID_SWITCH` (`string`)

- This SQL hint should not be used in general.

  It is needed by PECL/mysqlnd_ms to group cache entries for one statement but originating from different physical connections. If the hint is used connection settings such as user, hostname and charset are not considered for generating a cache key of a query. Instead the given value and the query string are used as input to the hashing function that generates the key.

  PECL/mysqlnd_ms may, if instructed, cache results from MySQL Replication slaves. Because it can hold many connections to the slave the cache key shall not be formed from the user, hostname or other settings that may vary for the various slave connections. Instead, PECL/mysqlnd_ms provides an identifier which refers to the group of slave connections that shall be enabled to share cache entries no matter which physical slave connection was to generate the cache entry.

  Use of this feature outside of PECL/mysqlnd_ms is not recommended.

`mysqlnd_qc_set_cache_condition` related

## Example `mysqlnd_qc_set_cache_condition` usage

The function `mysqlnd_qc_set_cache_condition` allows setting conditions for automatic caching of statements which don't begin with the SQL hints necessary to manually enable caching.

    <?php
    /* Cache all accesses to tables with the name "new%" in schema/database "db_example" for 1 second */
    if (!mysqlnd_qc_set_cache_condition(MYSQLND_QC_CONDITION_META_SCHEMA_PATTERN, "db_example.new%", 1)) {
      die("Failed to set cache condition!");
    }

    $mysqli = new mysqli("host", "user", "password", "db_example", "port");
    /* cached although no SQL hint given  */
    $mysqli->query("SELECT id, title FROM news");

    $pdo_mysql = new PDO("mysql:host=host;dbname=db_example;port=port", "user", "password");
    /* not cached: no SQL hint, no pattern match */
    $pdo_mysql->query("SELECT id, title FROM latest_news");
    /* cached: TTL 1 second, pattern match */
    $pdo_mysql->query("SELECT id, title FROM news");
    ?>

`MYSQLND_QC_CONDITION_META_SCHEMA_PATTERN` (`int`)

- Used as a parameter of `mysqlnd_qc_set_cache_condition` to set conditions for schema based automatic caching.

Other

The plugin version number can be obtained using either `MYSQLND_QC_VERSION`, which is the string representation of the numerical version number, or `MYSQLND_QC_VERSION_ID`, which is an integer such as 10000. Developers can calculate the version number as follows.

Version (part) | Example
Major*10000 | 1*10000 = 10000
Minor*100 | 0*100 = 0
Patch | 0 = 0
MYSQLND_QC_VERSION_ID | 10000

`MYSQLND_QC_VERSION` (`string`)

- Plugin version string, for example, 1.0.0-prototype.

`MYSQLND_QC_VERSION_ID` (`int`)

- Plugin version number, for example, 10000.