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Add note for index requirement with FK constraint 

Update migration_style_guide.md
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Shinya Maeda 2019-05-23 12:45:52 +00:00 committed by Marcia Ramos
parent 26d608cb60
commit ff14b58dc5
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doc/development/migration_style_guide.md
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@ -186,7 +186,11 @@ end
When adding a foreign-key constraint to either an existing or new
column remember to also add a index on the column.
This is _required_ for all foreign-keys.
This is **required** for all foreign-keys, e.g., to support efficient cascading
deleting: when a lot of rows in a table get deleted, the referenced records need
to be deleted too. The database has to look for corresponding records in the
referenced table. Without an index, this will result in a sequential scan on the
table which can take a long time.
Here's an example where we add a new column with a foreign key
constraint. Note it includes `index: true` to create an index for it.