Tables with many columns.
HOW THEY ARISE
Columns added over years, none removed.
WHAT PROBLEMS THEY CAUSE
Rows too large for efficient storage Reads pulling far more than needed Indexes that cannot cover queries Schema changes taking longer
WHAT TO LOOK FOR
Columns rarely populated Columns rarely read Groups of columns used only together
WHAT THAT LAST GROUP SUGGESTS
A separate table.
WHAT VERTICAL SPLITTING IS
Moving rarely used columns into a related table sharing the key.
WHAT IT PROVIDES
A narrow, frequently read main table.
WHAT IT COSTS
A join when the extra columns are needed.
WHEN THAT TRADE IS WORTH IT
When the main table is read constantly and the extras rarely.
WHAT TO BE CAREFUL WITH
Large text or binary columns stored inline.
WHY
They can be read even when not selected, depending on storage.
WHAT TO DO
Move them to a separate table, or out of the database.
WHAT TO MEASURE
Average row size, and how much is actually read.
WHAT TO AVOID
Splitting pre-emptively without evidence.
WHAT TO DO BEFORE ANY SPLIT
Confirm which queries would benefit, and by how much.
