Schema.php

<?php

namespace Drupal\Core\Database\Driver\mysql;

use Drupal\Core\Database\SchemaException;
use Drupal\Core\Database\SchemaObjectExistsException;
use Drupal\Core\Database\SchemaObjectDoesNotExistException;
use Drupal\Core\Database\Schema as DatabaseSchema;
use Drupal\Component\Utility\Unicode;

/**
 * @addtogroup schemaapi
 * @{
 */

/**
 * MySQL implementation of \Drupal\Core\Database\Schema.
 */
class Schema extends DatabaseSchema {
    
    /**
     * Maximum length of a table comment in MySQL.
     */
    const COMMENT_MAX_TABLE = 60;
    
    /**
     * Maximum length of a column comment in MySQL.
     */
    const COMMENT_MAX_COLUMN = 255;
    
    /**
     * @var array
     *   List of MySQL string types.
     */
    protected $mysqlStringTypes = [
        'VARCHAR',
        'CHAR',
        'TINYTEXT',
        'MEDIUMTEXT',
        'LONGTEXT',
        'TEXT',
    ];
    
    /**
     * Get information about the table and database name from the prefix.
     *
     * @return
     *   A keyed array with information about the database, table name and prefix.
     */
    protected function getPrefixInfo($table = 'default', $add_prefix = TRUE) {
        $info = [
            'prefix' => $this->connection
                ->tablePrefix($table),
        ];
        if ($add_prefix) {
            $table = $info['prefix'] . $table;
        }
        if (($pos = strpos($table, '.')) !== FALSE) {
            $info['database'] = substr($table, 0, $pos);
            $info['table'] = substr($table, ++$pos);
        }
        else {
            $info['database'] = $this->connection
                ->getConnectionOptions()['database'];
            $info['table'] = $table;
        }
        return $info;
    }
    
    /**
     * Build a condition to match a table name against a standard information_schema.
     *
     * MySQL uses databases like schemas rather than catalogs so when we build
     * a condition to query the information_schema.tables, we set the default
     * database as the schema unless specified otherwise, and exclude table_catalog
     * from the condition criteria.
     */
    protected function buildTableNameCondition($table_name, $operator = '=', $add_prefix = TRUE) {
        $table_info = $this->getPrefixInfo($table_name, $add_prefix);
        $condition = $this->connection
            ->condition('AND');
        $condition->condition('table_schema', $table_info['database']);
        $condition->condition('table_name', $table_info['table'], $operator);
        return $condition;
    }
    
    /**
     * Generate SQL to create a new table from a Drupal schema definition.
     *
     * @param $name
     *   The name of the table to create.
     * @param $table
     *   A Schema API table definition array.
     *
     * @return
     *   An array of SQL statements to create the table.
     */
    protected function createTableSql($name, $table) {
        $info = $this->connection
            ->getConnectionOptions();
        // Provide defaults if needed.
        $table += [
            'mysql_engine' => 'InnoDB',
            'mysql_character_set' => 'utf8mb4',
        ];
        $sql = "CREATE TABLE {" . $name . "} (\n";
        // Add the SQL statement for each field.
        foreach ($table['fields'] as $field_name => $field) {
            $sql .= $this->createFieldSql($field_name, $this->processField($field)) . ", \n";
        }
        // Process keys & indexes.
        if (!empty($table['primary key']) && is_array($table['primary key'])) {
            $this->ensureNotNullPrimaryKey($table['primary key'], $table['fields']);
        }
        $keys = $this->createKeysSql($table);
        if (count($keys)) {
            $sql .= implode(", \n", $keys) . ", \n";
        }
        // Remove the last comma and space.
        $sql = substr($sql, 0, -3) . "\n) ";
        $sql .= 'ENGINE = ' . $table['mysql_engine'] . ' DEFAULT CHARACTER SET ' . $table['mysql_character_set'];
        // By default, MySQL uses the default collation for new tables, which is
        // 'utf8mb4_general_ci' (MySQL 5) or 'utf8mb4_0900_ai_ci' (MySQL 8) for
        // utf8mb4. If an alternate collation has been set, it needs to be
        // explicitly specified.
        // @see \Drupal\Core\Database\Driver\mysql\Schema
        if (!empty($info['collation'])) {
            $sql .= ' COLLATE ' . $info['collation'];
        }
        // Add table comment.
        if (!empty($table['description'])) {
            $sql .= ' COMMENT ' . $this->prepareComment($table['description'], self::COMMENT_MAX_TABLE);
        }
        return [
            $sql,
        ];
    }
    
    /**
     * Create an SQL string for a field to be used in table creation or alteration.
     *
     * Before passing a field out of a schema definition into this function it has
     * to be processed by _db_process_field().
     *
     * @param string $name
     *   Name of the field.
     * @param array $spec
     *   The field specification, as per the schema data structure format.
     */
    protected function createFieldSql($name, $spec) {
        $sql = "`" . $name . "` " . $spec['mysql_type'];
        if (in_array($spec['mysql_type'], $this->mysqlStringTypes)) {
            if (isset($spec['length'])) {
                $sql .= '(' . $spec['length'] . ')';
            }
            if (isset($spec['type']) && $spec['type'] == 'varchar_ascii') {
                $sql .= ' CHARACTER SET ascii';
            }
            if (!empty($spec['binary'])) {
                $sql .= ' BINARY';
            }
            elseif (isset($spec['type']) && $spec['type'] == 'varchar_ascii') {
                $sql .= ' COLLATE ascii_general_ci';
            }
        }
        elseif (isset($spec['precision']) && isset($spec['scale'])) {
            $sql .= '(' . $spec['precision'] . ', ' . $spec['scale'] . ')';
        }
        if (!empty($spec['unsigned'])) {
            $sql .= ' unsigned';
        }
        if (isset($spec['not null'])) {
            if ($spec['not null']) {
                $sql .= ' NOT NULL';
            }
            else {
                $sql .= ' NULL';
            }
        }
        if (!empty($spec['auto_increment'])) {
            $sql .= ' auto_increment';
        }
        // $spec['default'] can be NULL, so we explicitly check for the key here.
        if (array_key_exists('default', $spec)) {
            $sql .= ' DEFAULT ' . $this->escapeDefaultValue($spec['default']);
        }
        if (empty($spec['not null']) && !isset($spec['default'])) {
            $sql .= ' DEFAULT NULL';
        }
        // Add column comment.
        if (!empty($spec['description'])) {
            $sql .= ' COMMENT ' . $this->prepareComment($spec['description'], self::COMMENT_MAX_COLUMN);
        }
        return $sql;
    }
    
    /**
     * Set database-engine specific properties for a field.
     *
     * @param $field
     *   A field description array, as specified in the schema documentation.
     */
    protected function processField($field) {
        if (!isset($field['size'])) {
            $field['size'] = 'normal';
        }
        // Set the correct database-engine specific datatype.
        // In case one is already provided, force it to uppercase.
        if (isset($field['mysql_type'])) {
            $field['mysql_type'] = mb_strtoupper($field['mysql_type']);
        }
        else {
            $map = $this->getFieldTypeMap();
            $field['mysql_type'] = $map[$field['type'] . ':' . $field['size']];
        }
        if (isset($field['type']) && $field['type'] == 'serial') {
            $field['auto_increment'] = TRUE;
        }
        return $field;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFieldTypeMap() {
        // Put :normal last so it gets preserved by array_flip. This makes
        // it much easier for modules (such as schema.module) to map
        // database types back into schema types.
        // $map does not use drupal_static as its value never changes.
        static $map = [
            'varchar_ascii:normal' => 'VARCHAR',
            'varchar:normal' => 'VARCHAR',
            'char:normal' => 'CHAR',
            'text:tiny' => 'TINYTEXT',
            'text:small' => 'TINYTEXT',
            'text:medium' => 'MEDIUMTEXT',
            'text:big' => 'LONGTEXT',
            'text:normal' => 'TEXT',
            'serial:tiny' => 'TINYINT',
            'serial:small' => 'SMALLINT',
            'serial:medium' => 'MEDIUMINT',
            'serial:big' => 'BIGINT',
            'serial:normal' => 'INT',
            'int:tiny' => 'TINYINT',
            'int:small' => 'SMALLINT',
            'int:medium' => 'MEDIUMINT',
            'int:big' => 'BIGINT',
            'int:normal' => 'INT',
            'float:tiny' => 'FLOAT',
            'float:small' => 'FLOAT',
            'float:medium' => 'FLOAT',
            'float:big' => 'DOUBLE',
            'float:normal' => 'FLOAT',
            'numeric:normal' => 'DECIMAL',
            'blob:big' => 'LONGBLOB',
            'blob:normal' => 'BLOB',
        ];
        return $map;
    }
    protected function createKeysSql($spec) {
        $keys = [];
        if (!empty($spec['primary key'])) {
            $keys[] = 'PRIMARY KEY (' . $this->createKeySql($spec['primary key']) . ')';
        }
        if (!empty($spec['unique keys'])) {
            foreach ($spec['unique keys'] as $key => $fields) {
                $keys[] = 'UNIQUE KEY `' . $key . '` (' . $this->createKeySql($fields) . ')';
            }
        }
        if (!empty($spec['indexes'])) {
            $indexes = $this->getNormalizedIndexes($spec);
            foreach ($indexes as $index => $fields) {
                $keys[] = 'INDEX `' . $index . '` (' . $this->createKeySql($fields) . ')';
            }
        }
        return $keys;
    }
    
    /**
     * Gets normalized indexes from a table specification.
     *
     * Shortens indexes to 191 characters if they apply to utf8mb4-encoded
     * fields, in order to comply with the InnoDB index limitation of 756 bytes.
     *
     * @param array $spec
     *   The table specification.
     *
     * @return array
     *   List of shortened indexes.
     *
     * @throws \Drupal\Core\Database\SchemaException
     *   Thrown if field specification is missing.
     */
    protected function getNormalizedIndexes(array $spec) {
        $indexes = isset($spec['indexes']) ? $spec['indexes'] : [];
        foreach ($indexes as $index_name => $index_fields) {
            foreach ($index_fields as $index_key => $index_field) {
                // Get the name of the field from the index specification.
                $field_name = is_array($index_field) ? $index_field[0] : $index_field;
                // Check whether the field is defined in the table specification.
                if (isset($spec['fields'][$field_name])) {
                    // Get the MySQL type from the processed field.
                    $mysql_field = $this->processField($spec['fields'][$field_name]);
                    if (in_array($mysql_field['mysql_type'], $this->mysqlStringTypes)) {
                        // Check whether we need to shorten the index.
                        if ((!isset($mysql_field['type']) || $mysql_field['type'] != 'varchar_ascii') && (!isset($mysql_field['length']) || $mysql_field['length'] > 191)) {
                            // Limit the index length to 191 characters.
                            $this->shortenIndex($indexes[$index_name][$index_key]);
                        }
                    }
                }
                else {
                    throw new SchemaException("MySQL needs the '{$field_name}' field specification in order to normalize the '{$index_name}' index");
                }
            }
        }
        return $indexes;
    }
    
    /**
     * Helper function for normalizeIndexes().
     *
     * Shortens an index to 191 characters.
     *
     * @param array $index
     *   The index array to be used in createKeySql.
     *
     * @see Drupal\Core\Database\Driver\mysql\Schema::createKeySql()
     * @see Drupal\Core\Database\Driver\mysql\Schema::normalizeIndexes()
     */
    protected function shortenIndex(&$index) {
        if (is_array($index)) {
            if ($index[1] > 191) {
                $index[1] = 191;
            }
        }
        else {
            $index = [
                $index,
                191,
            ];
        }
    }
    protected function createKeySql($fields) {
        $return = [];
        foreach ($fields as $field) {
            if (is_array($field)) {
                $return[] = '`' . $field[0] . '`(' . $field[1] . ')';
            }
            else {
                $return[] = '`' . $field . '`';
            }
        }
        return implode(', ', $return);
    }
    
    /**
     * {@inheritdoc}
     */
    public function renameTable($table, $new_name) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("Cannot rename '{$table}' to '{$new_name}': table '{$table}' doesn't exist.");
        }
        if ($this->tableExists($new_name)) {
            throw new SchemaObjectExistsException("Cannot rename '{$table}' to '{$new_name}': table '{$new_name}' already exists.");
        }
        $info = $this->getPrefixInfo($new_name);
        $this->connection
            ->query('ALTER TABLE {' . $table . '} RENAME TO `' . $info['table'] . '`');
    }
    
    /**
     * {@inheritdoc}
     */
    public function dropTable($table) {
        if (!$this->tableExists($table)) {
            return FALSE;
        }
        $this->connection
            ->query('DROP TABLE {' . $table . '}');
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function addField($table, $field, $spec, $keys_new = []) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("Cannot add field '{$table}.{$field}': table doesn't exist.");
        }
        if ($this->fieldExists($table, $field)) {
            throw new SchemaObjectExistsException("Cannot add field '{$table}.{$field}': field already exists.");
        }
        // Fields that are part of a PRIMARY KEY must be added as NOT NULL.
        $is_primary_key = isset($keys_new['primary key']) && in_array($field, $keys_new['primary key'], TRUE);
        if ($is_primary_key) {
            $this->ensureNotNullPrimaryKey($keys_new['primary key'], [
                $field => $spec,
            ]);
        }
        $fixnull = FALSE;
        if (!empty($spec['not null']) && !isset($spec['default']) && !$is_primary_key) {
            $fixnull = TRUE;
            $spec['not null'] = FALSE;
        }
        $query = 'ALTER TABLE {' . $table . '} ADD ';
        $query .= $this->createFieldSql($field, $this->processField($spec));
        if ($keys_sql = $this->createKeysSql($keys_new)) {
            // Make sure to drop the existing primary key before adding a new one.
            // This is only needed when adding a field because this method, unlike
            // changeField(), is supposed to handle primary keys automatically.
            if (isset($keys_new['primary key']) && $this->indexExists($table, 'PRIMARY')) {
                $query .= ', DROP PRIMARY KEY';
            }
            $query .= ', ADD ' . implode(', ADD ', $keys_sql);
        }
        $this->connection
            ->query($query);
        if (isset($spec['initial_from_field'])) {
            if (isset($spec['initial'])) {
                $expression = 'COALESCE(' . $spec['initial_from_field'] . ', :default_initial_value)';
                $arguments = [
                    ':default_initial_value' => $spec['initial'],
                ];
            }
            else {
                $expression = $spec['initial_from_field'];
                $arguments = [];
            }
            $this->connection
                ->update($table)
                ->expression($field, $expression, $arguments)
                ->execute();
        }
        elseif (isset($spec['initial'])) {
            $this->connection
                ->update($table)
                ->fields([
                $field => $spec['initial'],
            ])
                ->execute();
        }
        if ($fixnull) {
            $spec['not null'] = TRUE;
            $this->changeField($table, $field, $field, $spec);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function dropField($table, $field) {
        if (!$this->fieldExists($table, $field)) {
            return FALSE;
        }
        // When dropping a field that is part of a composite primary key MySQL
        // automatically removes the field from the primary key, which can leave the
        // table in an invalid state. MariaDB 10.2.8 requires explicitly dropping
        // the primary key first for this reason. We perform this deletion
        // explicitly which also makes the behavior on both MySQL and MariaDB
        // consistent with PostgreSQL.
        // @see https://mariadb.com/kb/en/library/alter-table
        $primary_key = $this->findPrimaryKeyColumns($table);
        if (count($primary_key) > 1 && in_array($field, $primary_key, TRUE)) {
            $this->dropPrimaryKey($table);
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} DROP `' . $field . '`');
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldSetDefault($table, $field, $default) {
        @trigger_error('fieldSetDefault() is deprecated in drupal:8.7.0 and will be removed before drupal:9.0.0. Instead, call ::changeField() passing a full field specification. See https://www.drupal.org/node/2999035', E_USER_DEPRECATED);
        if (!$this->fieldExists($table, $field)) {
            throw new SchemaObjectDoesNotExistException("Cannot set default value of field '{$table}.{$field}': field doesn't exist.");
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} ALTER COLUMN `' . $field . '` SET DEFAULT ' . $this->escapeDefaultValue($default));
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldSetNoDefault($table, $field) {
        @trigger_error('fieldSetNoDefault() is deprecated in drupal:8.7.0 and will be removed before drupal:9.0.0. Instead, call ::changeField() passing a full field specification. See https://www.drupal.org/node/2999035', E_USER_DEPRECATED);
        if (!$this->fieldExists($table, $field)) {
            throw new SchemaObjectDoesNotExistException("Cannot remove default value of field '{$table}.{$field}': field doesn't exist.");
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} ALTER COLUMN `' . $field . '` DROP DEFAULT');
    }
    
    /**
     * {@inheritdoc}
     */
    public function indexExists($table, $name) {
        // Returns one row for each column in the index. Result is string or FALSE.
        // Details at http://dev.mysql.com/doc/refman/5.0/en/show-index.html
        $row = $this->connection
            ->query('SHOW INDEX FROM {' . $table . '} WHERE key_name = ' . $this->connection
            ->quote($name))
            ->fetchAssoc();
        return isset($row['Key_name']);
    }
    
    /**
     * {@inheritdoc}
     */
    public function addPrimaryKey($table, $fields) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("Cannot add primary key to table '{$table}': table doesn't exist.");
        }
        if ($this->indexExists($table, 'PRIMARY')) {
            throw new SchemaObjectExistsException("Cannot add primary key to table '{$table}': primary key already exists.");
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} ADD PRIMARY KEY (' . $this->createKeySql($fields) . ')');
    }
    
    /**
     * {@inheritdoc}
     */
    public function dropPrimaryKey($table) {
        if (!$this->indexExists($table, 'PRIMARY')) {
            return FALSE;
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} DROP PRIMARY KEY');
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function findPrimaryKeyColumns($table) {
        if (!$this->tableExists($table)) {
            return FALSE;
        }
        $result = $this->connection
            ->query("SHOW KEYS FROM {" . $table . "} WHERE Key_name = 'PRIMARY'")
            ->fetchAllAssoc('Column_name');
        return array_keys($result);
    }
    
    /**
     * {@inheritdoc}
     */
    public function addUniqueKey($table, $name, $fields) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("Cannot add unique key '{$name}' to table '{$table}': table doesn't exist.");
        }
        if ($this->indexExists($table, $name)) {
            throw new SchemaObjectExistsException("Cannot add unique key '{$name}' to table '{$table}': unique key already exists.");
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} ADD UNIQUE KEY `' . $name . '` (' . $this->createKeySql($fields) . ')');
    }
    
    /**
     * {@inheritdoc}
     */
    public function dropUniqueKey($table, $name) {
        if (!$this->indexExists($table, $name)) {
            return FALSE;
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} DROP KEY `' . $name . '`');
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function addIndex($table, $name, $fields, array $spec) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("Cannot add index '{$name}' to table '{$table}': table doesn't exist.");
        }
        if ($this->indexExists($table, $name)) {
            throw new SchemaObjectExistsException("Cannot add index '{$name}' to table '{$table}': index already exists.");
        }
        $spec['indexes'][$name] = $fields;
        $indexes = $this->getNormalizedIndexes($spec);
        $this->connection
            ->query('ALTER TABLE {' . $table . '} ADD INDEX `' . $name . '` (' . $this->createKeySql($indexes[$name]) . ')');
    }
    
    /**
     * {@inheritdoc}
     */
    public function dropIndex($table, $name) {
        if (!$this->indexExists($table, $name)) {
            return FALSE;
        }
        $this->connection
            ->query('ALTER TABLE {' . $table . '} DROP INDEX `' . $name . '`');
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function introspectIndexSchema($table) {
        if (!$this->tableExists($table)) {
            throw new SchemaObjectDoesNotExistException("The table {$table} doesn't exist.");
        }
        $index_schema = [
            'primary key' => [],
            'unique keys' => [],
            'indexes' => [],
        ];
        $result = $this->connection
            ->query('SHOW INDEX FROM {' . $table . '}')
            ->fetchAll();
        foreach ($result as $row) {
            if ($row->Key_name === 'PRIMARY') {
                $index_schema['primary key'][] = $row->Column_name;
            }
            elseif ($row->Non_unique == 0) {
                $index_schema['unique keys'][$row->Key_name][] = $row->Column_name;
            }
            else {
                $index_schema['indexes'][$row->Key_name][] = $row->Column_name;
            }
        }
        return $index_schema;
    }
    
    /**
     * {@inheritdoc}
     */
    public function changeField($table, $field, $field_new, $spec, $keys_new = []) {
        if (!$this->fieldExists($table, $field)) {
            throw new SchemaObjectDoesNotExistException("Cannot change the definition of field '{$table}.{$field}': field doesn't exist.");
        }
        if ($field != $field_new && $this->fieldExists($table, $field_new)) {
            throw new SchemaObjectExistsException("Cannot rename field '{$table}.{$field}' to '{$field_new}': target field already exists.");
        }
        if (isset($keys_new['primary key']) && in_array($field_new, $keys_new['primary key'], TRUE)) {
            $this->ensureNotNullPrimaryKey($keys_new['primary key'], [
                $field_new => $spec,
            ]);
        }
        $sql = 'ALTER TABLE {' . $table . '} CHANGE `' . $field . '` ' . $this->createFieldSql($field_new, $this->processField($spec));
        if ($keys_sql = $this->createKeysSql($keys_new)) {
            $sql .= ', ADD ' . implode(', ADD ', $keys_sql);
        }
        $this->connection
            ->query($sql);
    }
    
    /**
     * {@inheritdoc}
     */
    public function prepareComment($comment, $length = NULL) {
        // Truncate comment to maximum comment length.
        if (isset($length)) {
            // Add table prefixes before truncating.
            $comment = Unicode::truncate($this->connection
                ->prefixTables($comment), $length, TRUE, TRUE);
        }
        // Remove semicolons to avoid triggering multi-statement check.
        $comment = strtr($comment, [
            ';' => '.',
        ]);
        return $this->connection
            ->quote($comment);
    }
    
    /**
     * Retrieve a table or column comment.
     */
    public function getComment($table, $column = NULL) {
        $condition = $this->buildTableNameCondition($table);
        if (isset($column)) {
            $condition->condition('column_name', $column);
            $condition->compile($this->connection, $this);
            // Don't use {} around information_schema.columns table.
            return $this->connection
                ->query("SELECT column_comment AS column_comment FROM information_schema.columns WHERE " . (string) $condition, $condition->arguments())
                ->fetchField();
        }
        $condition->compile($this->connection, $this);
        // Don't use {} around information_schema.tables table.
        $comment = $this->connection
            ->query("SELECT table_comment AS table_comment FROM information_schema.tables WHERE " . (string) $condition, $condition->arguments())
            ->fetchField();
        // Work-around for MySQL 5.0 bug http://bugs.mysql.com/bug.php?id=11379
        return preg_replace('/; InnoDB free:.*$/', '', $comment);
    }
    
    /**
     * {@inheritdoc}
     */
    public function tableExists($table) {
        // The information_schema table is very slow to query under MySQL 5.0.
        // Instead, we try to select from the table in question.  If it fails,
        // the most likely reason is that it does not exist. That is dramatically
        // faster than using information_schema.
        // @link http://bugs.mysql.com/bug.php?id=19588
        // @todo This override should be removed once we require a version of MySQL
        //   that has that bug fixed.
        try {
            $this->connection
                ->queryRange("SELECT 1 FROM {" . $table . "}", 0, 1);
            return TRUE;
        } catch (\Exception $e) {
            return FALSE;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldExists($table, $column) {
        // The information_schema table is very slow to query under MySQL 5.0.
        // Instead, we try to select from the table and field in question. If it
        // fails, the most likely reason is that it does not exist. That is
        // dramatically faster than using information_schema.
        // @link http://bugs.mysql.com/bug.php?id=19588
        // @todo This override should be removed once we require a version of MySQL
        //   that has that bug fixed.
        try {
            $this->connection
                ->queryRange("SELECT {$column} FROM {" . $table . "}", 0, 1);
            return TRUE;
        } catch (\Exception $e) {
            return FALSE;
        }
    }

}

/**
 * @} End of "addtogroup schemaapi".
 */