index分析，可在操作链中指定需要强制使用的索引
@access protected
@param mixed $index
@return string
160927
protected function parseForce($index) {
        if(empty($index)) return '';
        if(is_array($index)) $index = join(",", $index);
        return sprintf(" FORCE INDEX ( %s ) ", $index);
    }
1352458
public function get($index){
		if(isset($this->map[ $index ])){
			return $this->map[ $index ];
		}
		return null;
	}
525403
protected function getIndexSqlDefinition(Index $index)
    {
        $def = '';
        $limit = '';
        if ($index->getLimit()) {
            $limit = '(' . $index->getLimit() . ')';
        }

        if ($index->getType() == Index::UNIQUE) {
            $def .= ' UNIQUE';
        }

        if ($index->getType() == Index::FULLTEXT) {
            $def .= ' FULLTEXT';
        }

        $def .= ' KEY';

        if (is_string($index->getName())) {
            $def .= ' `' . $index->getName() . '`';
        }

        $def .= ' (`' . implode('`,`', $index->getColumns()) . '`' . $limit . ')';

        return $def;
    }
369925
public function item(int $index): ?string
    {
        $classes = $this->getClasses();

        return $classes[$index] ?? null;
    }
439772
public function getRow($index)
    {
        $keys = array_keys($this->rows);

        return isset($keys[$index]) ? $this->rows[$keys[$index]] : null;
    }
407675
public function getName($index = null)
    {
        $type = $this->getType($index);
        return is_int($type) ? token_name($type) : null;
    }
1218368
protected function readMsgId($index)
    {
        $msgId = $this->readStringFromTable($index, $this->msgIdTable);
        if (false === $msgId) {
            $msgId = array('');
        }

        return $msgId;
    }
57558
public function getRow($index)
    {
        return isset($this->rows[$index]) ? $this->rows[$index] : null;
    }
1101394
public function get(string $index) : string
    {
        if (!$this->isValid($index)) {
            throw new \InvalidArgumentException('The specified value does not exist');
        }
        return $this->map[$index];
    }
919849
protected function getSortingPart($index)
    {
        $sorting = $this->getSorting();
        if ($sorting === '') {
            return null;
        }

        $parts = explode(' ', $sorting);
        return isset($parts[$index]) ? $parts[$index] : null;
    }
1304453
public function getSearchIndex($index)
    {
        return isset($this->search[$index]) ? $this->search[$index] : null;
    }
1201026
public function getLineString($index)
    {
        if ($index == -1) {
            $index = count($this->lineStrings) - 1;
        }

        $lineStringClass = $this->getNamespace() . '\LineString';

        return new $lineStringClass($this->lineStrings[$index], $this->srid);
    }
620469
public function getParameter($index)
    {
        if (!is_string($index) && !is_numeric($index)) {
            return null;
        }
        if (array_key_exists($index, $this->_parameterNameMap)) {
            $index = $this->_parameterNameMap[$index];
        }
        if (array_key_exists($index, $this->_parameters)) {
            return $this->_parameters[$index];
        }
        return null;
    }
508272
public function segment($index) {
		if (isset($this->segments[$index])) {
			return $this->segments[$index];
		}

		return null;
	}
550361
protected function getPrefixAt($index)
    {
        $prefixes = $this->getPrefixes();
        return isset($prefixes[$index]) ? $prefixes[$index] : null;
    }
417753
protected function itemAt($index)
    {
        if (isset($this->_data[$index])) {
            return $this->_data[$index];
        } elseif ($index >= 0 && $index < $this->_count) {
            return $this->_data[$index];
        } else {
            throw new Exception(str_cover('List索引"{index}"已超出范围', [
                '{index}' => $index,
            ]), 100100507);
        }
    }
1082086
public function getKey($index)
    {
        Assertion::integer($index, 'The index must be a positive integer.');
        Assertion::greaterOrEqualThan($index, 0, 'The index must be a positive integer.');
        Assertion::true($this->hasKey($index), 'Undefined index.');

        return $this->getKeys()[$index];
    }