diff options
Diffstat (limited to 'doc/development/what_requires_downtime.md')
| -rw-r--r-- | doc/development/what_requires_downtime.md | 25 |
1 files changed, 5 insertions, 20 deletions
diff --git a/doc/development/what_requires_downtime.md b/doc/development/what_requires_downtime.md index f0da1cc2ddc..944bf5900c5 100644 --- a/doc/development/what_requires_downtime.md +++ b/doc/development/what_requires_downtime.md @@ -7,9 +7,8 @@ downtime. ## Adding Columns -On PostgreSQL you can safely add a new column to an existing table as long as it -does **not** have a default value. For example, this query would not require -downtime: +You can safely add a new column to an existing table as long as it does **not** +have a default value. For example, this query would not require downtime: ```sql ALTER TABLE projects ADD COLUMN random_value int; @@ -27,11 +26,6 @@ This requires updating every single row in the `projects` table so that indexes in a table. This in turn acquires enough locks on the table for it to effectively block any other queries. -As of MySQL 5.6 adding a column to a table is still quite an expensive -operation, even when using `ALGORITHM=INPLACE` and `LOCK=NONE`. This means -downtime _may_ be required when modifying large tables as otherwise the -operation could potentially take hours to complete. - Adding a column with a default value _can_ be done without requiring downtime when using the migration helper method `Gitlab::Database::MigrationHelpers#add_column_with_default`. This method works @@ -311,8 +305,7 @@ migrations](background_migrations.md#cleaning-up). ## Adding Indexes Adding indexes is an expensive process that blocks INSERT and UPDATE queries for -the duration. When using PostgreSQL one can work around this by using the -`CONCURRENTLY` option: +the duration. You can work around this by using the `CONCURRENTLY` option: ```sql CREATE INDEX CONCURRENTLY index_name ON projects (column_name); @@ -336,17 +329,9 @@ end Note that `add_concurrent_index` can not be reversed automatically, thus you need to manually define `up` and `down`. -When running this on PostgreSQL the `CONCURRENTLY` option mentioned above is -used. On MySQL this method produces a regular `CREATE INDEX` query. - -MySQL doesn't really have a workaround for this. Supposedly it _can_ create -indexes without the need for downtime but only for variable width columns. The -details on this are a bit sketchy. Since it's better to be safe than sorry one -should assume that adding indexes requires downtime on MySQL. - ## Dropping Indexes -Dropping an index does not require downtime on both PostgreSQL and MySQL. +Dropping an index does not require downtime. ## Adding Tables @@ -370,7 +355,7 @@ transaction this means this approach would require downtime. GitLab allows you to work around this by using `Gitlab::Database::MigrationHelpers#add_concurrent_foreign_key`. This method -ensures that when PostgreSQL is used no downtime is needed. +ensures that no downtime is needed. ## Removing Foreign Keys |