Warning: Undefined array key "type" in /vendor/magento/framework/Setup/Declaration/Schema/Db/SchemaBuilder.php on line 90

This error occurs when a table column is declared in the db_schema.xml file without specifying the required attribute: the column type (xsi:type). Magento 2.4.8, along with 2.4.7-p5, expects this attribute to properly interpret and generate the database schema.

Example of incorrect declaration
In the example below, the column is defined without specifying the type (xsi:type), which causes the error:

<?xml version="1.0"?>
<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">
    <table name="your_table_name" resource="default" engine="innodb" comment="Table Comment">
        <!-- additional declaration -->
        <column name="code" nullable="true" length="32" comment="Code"/>
    </table>
</schema>

Correct declaration with xsi:type
To resolve this issue, ensure that the xsi:type attribute is properly defined for each column. In the case of a varchar column, the correct declaration would look like this:

<?xml version="1.0"?>
<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">
    <table name="your_table_name" resource="default" engine="innodb" comment="Table Comment">
        <!-- additional declaration -->
        <column xsi:type="varchar" name="code" nullable="true" length="32" comment="Code"/>
    </table>
</schema>

For tables that are defined without columns and marked with disabled="true"

<table name="test_table" resource="default" engine="innodb" comment="Table" disabled="true">
</table>

How to troubleshoot and Identify the Issue
If you are encountering this error and want to track down which columns or tables are affected, you can modify the vendor/magento/framework/Setup/Declaration/Schema/Db/SchemaBuilder.php file for more detailed error reporting. By adding additional error handling, you can pinpoint the table and column where the issue occurs.

Original code:

if ($columnData['type'] == 'json') {
    $tablesWithJsonTypeField[$keyTable] = $keyColumn;
}

Replace with:

try {
    if ($columnData['type'] == 'json') {
        $tablesWithJsonTypeField[$keyTable] = $keyColumn;
    }
} catch (\Throwable $e) {
    // Create a new exception with extended context message
    $errorMessage = sprintf(
        "%s\nError processing table %s column %s",
        $e->getMessage(),
        $keyTable,
        $keyColumn
    );

    // Throw a new exception with the extended message
    // This preserves the original error but adds our context
    throw new \Exception($errorMessage, $e->getCode(), $e);
}

More resources:

• Pull request: Improve error handling SchemaBuilder: https://github.com/magento/magento2/pull/39799
• Patch file for the pull request: https://patch-diff.githubusercontent.com/raw/magento/magento2/pull/39799.patch

Looks like one of the custom modules dbschema.xml file missing "type" attribute with one of the columns definition. Here is how you can troubleshoot and resolve this issue:

1. Check the db_schema.xml files in all custom and third-party modules. Look for any column definitions that might be missing the type attribute.

2. Run the command with debugging enabled: php bin/magento setup:upgrade -vvv. This might give you more context about which module or file is causing the issue.

3. Ensure that all db_schema.xml files follow Magento's schema declaration standards. Every column should have a type attribute defined.