Apex Heap Size Too Large: 29,514,682 Bytes

This error usually occurs when Apex retrieves too many records, too many fields, or both.

The Apex heap size limit for a synchronous transaction is 6 MB.

To reduce heap usage:

• Query only the records that are actually needed.
• Select only the fields required for processing.
• Avoid selecting unnecessary relationship fields.
• Relationship data is repeated for every returned row, which can significantly increase memory usage.
• When many records reference the same related record, query the related records separately and store them in a Map<Id, SObject>.

For example, instead of selecting many parent fields in the main query:

SELECT Id, Account.Name, Account.BillingCity, Account.BillingState
FROM Contact

Query only the parent ID:

SELECT Id, AccountId
FROM Contact

Then retrieve the required Account data separately and store it in a map:

Map<Id, Account> accountMap = new Map<Id, Account>(
    [SELECT Id, Name, BillingCity, BillingState
     FROM Account
     WHERE Id IN :accountIds]
);

This prevents the same relationship data from being stored repeatedly for every row and can significantly reduce heap usage.