Is there any reason to worry about the column order in a table?

Question

I know you can ALTER the column order in MySQL with FIRST and AFTER, but why would you want to bother? Since good queries explicitly name columns when inserting data, is the

Answer 1

As noted, there are numerous potential performance issues. I once worked on a database where putting very large columns at the end improved performance if you didn't reference those columns in your query. Apparently if a record spanned multiple disk blocks, the database engine could stop reading blocks once it got all the columns it needed.

Of course any performance implications are highly dependent not just on the manufacturer that you're using, but also potentially on the version. A few months ago I noticed that our Postgres could not use an index for a "like" comparison. That is, if you wrote "somecolumn like 'M%'", it wasn't smart enough to skip to the M's and quit when it found the first N. I was planning to change a bunch of queries to use "between". Then we got a new version of Postgres and it handled the like's intelligently. Glad I never got around to changing the queries. Obviously not directly relevant here but my point is that anything you do for efficiency considerations could be obsolete with the next version.

Column order is almost always very relevant to me because I routinely write generic code that reads the database schema to create screens. Like, my "edit a record" screens are almost always built by reading the schema to get the list of fields, and then displaying them in order. If I changed the order of columns, my program would still work, but the display might be strange to the user. Like, you expect to see name / address / city / state / zip, not city / address / zip / name / state. Sure, I could put the display order of the columns in code or a control file or something, but then every time we added or removed a column we'd have to remember to go update the control file. I like to say things once. Also, when the edit screen is built purely from the schema, adding a new table can mean writing zero lines of code to create an edit screen for it, which is way cool. (Well, okay, in practice usually I have to add an entry to the menu to call the generic edit program, and I've generally given up on generic "select a record to update" because there are too many exceptions to make it practical.)