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Diffstat (limited to 'doc/development/migration_style_guide.md')
| -rw-r--r-- | doc/development/migration_style_guide.md | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/doc/development/migration_style_guide.md b/doc/development/migration_style_guide.md index 0c326eeb851..9b26f691b55 100644 --- a/doc/development/migration_style_guide.md +++ b/doc/development/migration_style_guide.md @@ -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. |