Hitting column limit error in MySQL when unioning large views

Other Questions MySQL
1

I’m working on a project where I need to merge two large views, each having 1029 columns. They look identical, so I used UNION ALL to combine them. Oddly enough, querying each view individually works fine, but when I merge them, I hit a ‘too many columns’ error even though the union should still result in 1029 columns.

I created a view like this:

CREATE OR REPLACE VIEW ViewX AS
SELECT * FROM ViewA
UNION ALL
SELECT * FROM ViewB;

The view gets created without issues, yet when I run:

SELECT ID FROM ViewX LIMIT 1

I receive Error 1117: Too many columns. Interestingly, if I run the UNION query directly without creating a view, it works. What could be causing this discrepancy and how can I resolve it?

2

This issue with the union of large views is intriguing. My experience with MySQL suggests that when a view is created, it involves handling the underlying query in a way that can expose limitations not seen in direct query execution. One possible cause is the internal materialization process, which may enforce stricter limits on the number of columns. A viable workaround is to avoid SELECT * by explicitly listing the columns. Alternatively, using a temporary table might bypass these constraints. Reviewing the design to reduce the number of columns could also be beneficial in the long run.

3

I’ve encountered similar issues when working with large datasets in MySQL. One workaround that’s worked for me is breaking down the view into smaller, more manageable chunks. Instead of using UNION ALL on the entire set of columns, try creating separate views for subsets of columns and then combining those. Something like:

CREATE VIEW ViewX_Part1 AS
SELECT col1, col2, …, col500 FROM ViewA
UNION ALL
SELECT col1, col2, …, col500 FROM ViewB;

CREATE VIEW ViewX_Part2 AS
SELECT col501, col502, …, col1029 FROM ViewA
UNION ALL
SELECT col501, col502, …, col1029 FROM ViewB;

Then you can query these partial views as needed. It’s not ideal, but it’s gotten me past similar column limits. Also, consider if you really need all 1029 columns in one view - that’s a lot to manage and might indicate a need for some schema optimization.