Craft CMS content migration to add a database index to custom fields. Also has a `safeDown()` method to remove the indexes

m231102_074933_add_index_to_demo_data.php

<?php

namespace craft\contentmigrations;

use Craft;
use craft\db\Migration;
use craft\db\Table;
use craft\helpers\ElementHelper;

/**
 * m231102_074933_add_index_to_demo_data migration.
 */
class m231102_074933_add_index_to_demo_data extends Migration
{
    // The field handles we want to add database indexes for
    public const FIELD_HANDLES = [
        'demoData'
    ];

    /**
     * @inheritdoc
     */
    public function safeUp(): bool
    {
        $fields = Craft::$app->getFields();
        // Iterate through all of our fields by handle
        foreach (self::FIELD_HANDLES as $fieldHandle) {
            $field = $fields->getFieldByHandle($fieldHandle);
            if ($field) {
                // Find the column name the field uses in the content table
                $column = ElementHelper::fieldColumnFromField($field);
                if ($column) {
                    // Create an index for this field if it isn't already created
                    $this->createIndexIfMissing(Table::CONTENT, [$column], false);
                }
            }
        }
        return true;
    }

    /**
     * @inheritdoc
     */
    public function safeDown(): bool
    {
        $fields = Craft::$app->getFields();
        // Iterate through all of our fields by handle
        foreach (self::FIELD_HANDLES as $fieldHandle) {
            $field = $fields->getFieldByHandle($fieldHandle);
            if ($field) {
                // Find the column name the field uses in the content table
                $column = ElementHelper::fieldColumnFromField($field);
                if ($column) {
                    // Delete the index for this field if it exists
                    $this->dropIndexIfExists(Table::CONTENT, [$column], false);
                }
            }
        }
        return true;
    }
}