Note: This was originally posted by an inactive account. Content was preserved by moving under an admin account.
Originally posted by: awilliams1024Another reason may be that you are hitting the limit on the length of the SOQL statement:
• The maximum query length on REST is bound by the URL limit imposed by our app servers, around 20,000 characters.
• The maximum query length is likely to be smaller than 20,000 characters because SOQL queries get translated and augmented with more information needed by the server side to execute the query (e.g. user name, org, complex formulas showing relationships etc.)
If you need to have all 800 columns maybe you could modify your data flow to pull subsets of the data in two separate queries and then join the data sets together using a unique identifier.