# `Ecto.Migration`
[🔗](https://github.com/elixir-ecto/ecto_sql/blob/v3.14.0/lib/ecto/migration.ex#L1)

Migrations are used to modify your database schema over time.

This module provides many helpers for migrating the database,
allowing developers to use Elixir to alter their storage in
a way that is database independent.

Migrations typically provide two operations: `up` and `down`,
allowing us to migrate the database forward or roll it back
in case of errors.

In order to manage migrations, Ecto creates a table called
`schema_migrations` in the database, which stores all migrations
that have already been executed. You can configure the name of
this table with the `:migration_source` configuration option
and the name of the repository that manages it with `:migration_repo`.

Ecto locks the `schema_migrations` table when running
migrations, guaranteeing two different servers cannot run the same
migration at the same time.

## Creating your first migration

Migrations are defined inside the "priv/REPO/migrations" where REPO
is the last part of the repository name in underscore. For example,
migrations for `MyApp.Repo` would be found in "priv/repo/migrations".
For `MyApp.CustomRepo`, it would be found in "priv/custom_repo/migrations".

Each file in the migrations directory has the following structure:

```text
NUMBER_NAME.exs
```

The NUMBER is a unique number that identifies the migration. It is
usually the timestamp of when the migration was created. The NAME
must also be unique and it quickly identifies what the migration
does. For example, if you need to track the "weather" in your system,
you can start a new file at "priv/repo/migrations/20190417140000_add_weather_table.exs"
that will have the following contents:

    defmodule MyRepo.Migrations.AddWeatherTable do
      use Ecto.Migration

      def up do
        create table("weather") do
          add :city,    :string, size: 40
          add :temp_lo, :integer
          add :temp_hi, :integer
          add :prcp,    :float

          timestamps()
        end
      end

      def down do
        drop table("weather")
      end
    end

The `up/0` function is responsible to migrate your database forward.
the `down/0` function is executed whenever you want to rollback.
The `down/0` function must always do the opposite of `up/0`.
Inside those functions, we invoke the API defined in this module,
you will find conveniences for managing tables, indexes, columns,
references, as well as running custom SQL commands.

To run a migration, we generally use Mix tasks. For example, you can
run the migration above by going to the root of your project and
typing:

    $ mix ecto.migrate

You can also roll it back by calling:

    $ mix ecto.rollback --step 1

Note rollback requires us to say how much we want to rollback.
On the other hand, `mix ecto.migrate` will always run all pending
migrations.

In practice, we don't create migration files by hand either, we
typically use `mix ecto.gen.migration` to generate the file with
the proper timestamp and then we just fill in its contents:

    $ mix ecto.gen.migration add_weather_table

For the rest of this document, we will cover the migration APIs
provided by Ecto. For a in-depth discussion of migrations and how
to use them safely within your application and data, see the
[Safe Ecto Migrations guide](safe_migrations.md).

## Mix tasks

As seen above, Ecto provides many Mix tasks to help developers work
with migrations. We summarize them below:

  * `mix ecto.gen.migration` - generates a
    migration that the user can fill in with particular commands
  * `mix ecto.migrate` - migrates a repository
  * `mix ecto.migrations` - shows all migrations and their status
  * `mix ecto.rollback` - rolls back a particular migration

Run `mix help COMMAND` for more information on a particular command.
For a lower level API for running migrations, see `Ecto.Migrator`.

## Change

Having to write both `up/0` and `down/0` functions for every
migration is tedious and error prone. For this reason, Ecto allows
you to define a `change/0` callback with all of the code you want
to execute when migrating and Ecto will automatically figure out
the `down/0` for you. For example, the migration above can be
written as:

    defmodule MyRepo.Migrations.AddWeatherTable do
      use Ecto.Migration

      def change do
        create table("weather") do
          add :city,    :string, size: 40
          add :temp_lo, :integer
          add :temp_hi, :integer
          add :prcp,    :float

          timestamps()
        end
      end
    end

However, note that not all commands are reversible. Trying to rollback
a non-reversible command will raise an `Ecto.MigrationError`.

A notable command in this regard is `execute/2`, which is reversible in
`change/0` by accepting a pair of plain SQL strings. The first is run on
forward migrations (`up/0`) and the second when rolling back (`down/0`).

If `up/0` and `down/0` are implemented in a migration, they take precedence,
and `change/0` isn't invoked.

## Field Types

The [Ecto primitive types](https://hexdocs.pm/ecto/Ecto.Schema.html#module-primitive-types) are mapped to the appropriate database
type by the various database adapters. For example, `:string` is
converted to `:varchar`, `:binary` to `:bytea` or `:blob`, and so on.

In particular, note that:

  * the `:string` type in migrations by default has a limit of 255 characters.
    If you need more or less characters, pass the `:size` option, such
    as `add :field, :string, size: 10`. If you don't want to impose a limit,
    most databases support a `:text` type or similar

  * the `:binary` type in migrations by default has no size limit. If you want
    to impose a limit, pass the `:size` option accordingly. In MySQL, passing
    the size option changes the underlying field from "blob" to "varbinary"

Any other type will be given as is to the database. For example, you
can use `:text`, `:char`, or `:varchar` as types. Types that have spaces
in their names can be wrapped in double quotes, such as `:"int unsigned"`,
`:"time without time zone"`, etc.

## Executing and flushing

Most functions in this module, when executed inside of migrations, are not
executed immediately. Instead they are performed after the relevant `up`,
`change`, or `down` callback terminates. Any other functions, such as
functions provided by `Ecto.Repo`, will be executed immediately unless they
are called from within an anonymous function passed to `execute/1`.

In some situations you may want to guarantee that all of the previous steps
have been executed before continuing. This is useful when you need to apply a
set of changes to the table before continuing with the migration. This can be
done with `flush/0`:

    def up do
      ...
      flush()
      ...
    end

However `flush/0` will raise if it would be called from `change` function when doing a rollback.
To avoid that we recommend to use `execute/2` with anonymous functions instead.
For more information and example usage please take a look at `execute/2` function.

## Formatter configuration

To enable Ecto's custom `mix format` rules in your migrations, you can create a new formatter
config file in your project called `priv/[your_repo]/migrations/.formatter.exs` with the
following content:

```elixir
[
  import_deps: [:ecto_sql],
  inputs: ["*.exs"]
]
```

You will also need to add a line or two to your project's main formatter config so that the
formatter knows where to find the new config file. Update (or create) your project's main
`.formatter.exs` file:

```elixir
[
  # Add this line to enable Ecto formatter rules
  import_deps: [:ecto],

  # Add this line to enable Ecto's formatter rules in your migrations directory
  subdirectories: ["priv/*/migrations"],

  # Default Elixir project rules
  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
]
```

Now, when you run `mix format`, the formatter should apply Ecto's custom rules when formatting
your migrations (e.g. no brackets are automatically added when creating columns with `add/3`).

## Repo configuration

### Migrator configuration

These options configure where Ecto stores and how Ecto runs your migrations:

  * `:migration_source` - Version numbers of migrations will be saved in a
    table named `schema_migrations` by default. You can configure the name of
    the table via:

        config :app, App.Repo, migration_source: "my_migrations"

  * `:migration_lock` - By default, Ecto will lock the migration source to throttle
    multiple nodes to run migrations one at a time. You can disable the `migration_lock`
    by setting it to `false`. You may also select a different locking strategy if
    supported by the adapter. See the adapter docs for more information.

        config :app, App.Repo, migration_lock: false

        # Or use a different locking strategy. For example, Postgres can use advisory
        # locks but be aware that your database configuration might not make this a good
        # fit. See the Ecto.Adapters.Postgres for more information:
        config :app, App.Repo, migration_lock: :pg_advisory_lock

  * `:migration_repo` - The migration repository is where the table managing the
    migrations will be stored (`migration_source` defines the table name). It defaults
    to the given repository itself but you can configure it via:

        config :app, App.Repo, migration_repo: App.MigrationRepo

  * `:migration_cast_version_column` - Ecto uses a `version` column of type
    `bigint` for the underlying migrations table (usually `schema_migrations`). By
    default, Ecto doesn't cast this to a different type when reading or writing to
    the database when running migrations. However, some web frameworks store this
    column as a string. For compatibility reasons, you can set this option to `true`,
    which makes Ecto perform a `CAST(version AS int)`. This used to be the default
    behavior up to Ecto 3.10, so if you are upgrading to 3.11+ and want to keep the
    old behavior, set this option to `true`.

  * `:priv` - the priv directory for the repo with the location of important assets,
    such as migrations. For a repository named `MyApp.FooRepo`, `:priv` defaults to
    "priv/foo_repo" and migrations should be placed at "priv/foo_repo/migrations"

  * `:start_apps_before_migration` - A list of applications to be started before
    running migrations. Used by `Ecto.Migrator.with_repo/3` and the migration tasks:

        config :app, App.Repo, start_apps_before_migration: [:ssl, :some_custom_logger]

### Migrations configuration

These options configure the default values used by migrations. **It is generally
discouraged to change any of those configurations after your database is deployed
to production, as changing these options will retroactively change how all
migrations work**.

  * `:migration_primary_key` - By default, Ecto uses the `:id` column with type
    `:bigserial`, but you can configure it via:

        config :app, App.Repo, migration_primary_key: [name: :uuid, type: :binary_id]

        config :app, App.Repo, migration_primary_key: false

    For Postgres version >= 10 `:identity` key may be used.
    By default, all :identity column will be bigints. You may provide optional
    parameters for `:start_value` and `:increment` to customize the created
    sequence. Config example:

        config :app, App.Repo, migration_primary_key: [type: :identity]

  * `:migration_foreign_key` - By default, Ecto uses the `primary_key` type
    for foreign keys when `references/2` is used, but you can configure it via:

        config :app, App.Repo, migration_foreign_key: [column: :uuid, type: :binary_id]

  * `:migration_timestamps` - By default, Ecto uses the `:naive_datetime` as the type,
    `:inserted_at` as the name of the column for storing insertion times, `:updated_at` as
    the name of the column for storing last-updated-at times, but you can configure it
    via:

        config :app, App.Repo, migration_timestamps: [
          type: :utc_datetime,
          inserted_at: :created_at,
          updated_at: :changed_at
        ]

    See `timestamps/1` for all supported options.

  * `:migration_default_prefix` - Ecto defaults to `nil` for the database prefix for
    migrations, but you can configure it via:

        config :app, App.Repo, migration_default_prefix: "my_prefix"

## Collations

Collations can be set on a column with the option `:collation`. This can be
useful when relying on ASCII sorting of characters when using a fractional index
for example. All supported collations and types that support setting a collocation
are not known by `ecto_sql` and specifying an incorrect collation or a collation on
an unsupported type might cause a migration to fail. Be sure to match the collation
on any column that references another column.

    def change do
      create table(:collate_reference) do
        add :name, :string, collation: "POSIX"
      end

      create table(:collate) do
        add :string, :string, collation: "POSIX"
        add :name_ref, references(:collate_reference, type: :string, column: :name), collation: "POSIX"
      end
    end

## Comments

Migrations where you create or alter a table support specifying table
and column comments. The same can be done when creating constraints
and indexes. Not all databases support this feature.

    def up do
      create index("posts", [:name], comment: "Index Comment")
      create constraint("products", "price_must_be_positive", check: "price > 0", comment: "Constraint Comment")
      create table("weather", prefix: "north_america", comment: "Table Comment") do
        add :city, :string, size: 40, comment: "Column Comment"
        timestamps()
      end
    end

## Prefixes

Migrations support specifying a table prefix or index prefix which will
target either a schema (if using PostgreSQL) or a different database (if using
MySQL). If no prefix is provided, the default schema or database is used.

Any reference declared in the table migration refers by default to the table
with the same declared prefix. The prefix is specified in the table options:

    def up do
      create table("weather", prefix: "north_america") do
        add :city,    :string, size: 40
        add :temp_lo, :integer
        add :temp_hi, :integer
        add :prcp,    :float
        add :group_id, references(:groups)

        timestamps()
      end

      create index("weather", [:city], prefix: "north_america")
    end

Note: if using MySQL with a prefixed table, you must use the same prefix
for the references since cross-database references are not supported.

When using a prefixed table with either MySQL or PostgreSQL, you must use the
same prefix for the index field to ensure that you index the prefix-qualified
table.

## Transaction Callbacks

If possible, each migration runs inside a transaction. This is true for Postgres,
but not true for MySQL, as the latter does not support DDL transactions.

In some rare cases, you may need to execute some common behavior after beginning
a migration transaction, or before committing that transaction. For instance, one
might desire to set a `lock_timeout` for each lock in the migration transaction.

You can do so by defining `c:after_begin/0` and `c:before_commit/0` callbacks to
your migration.

However, if you need do so for every migration module, implement this callback
for every migration can be quite repetitive. Luckily, you can handle this by
providing your migration module:

    defmodule MyApp.Migration do
      defmacro __using__(_) do
        quote do
          use Ecto.Migration

          def after_begin() do
            repo().query! "SET lock_timeout TO '5s'"
          end
        end
      end
    end

Then in your migrations you can `use MyApp.Migration` to share this behavior
among all your migrations.

## Additional resources

  * The [Safe Ecto Migrations guide](safe_migrations.md)

# `after_begin`
*optional* 

```elixir
@callback after_begin() :: term()
```

Migration code to run immediately after the transaction is opened.

Keep in mind that it is treated like any normal migration code, and should
consider both the up *and* down cases of the migration.

# `before_commit`
*optional* 

```elixir
@callback before_commit() :: term()
```

Migration code to run immediately before the transaction is closed.

Keep in mind that it is treated like any normal migration code, and should
consider both the up *and* down cases of the migration.

# `add`

Adds a column when creating or altering a table.

This function also accepts Ecto primitive types as column types
that are normalized by the database adapter. For example,
`:string` is converted to `:varchar`, `:binary` to `:bits` or `:blob`,
and so on.

However, the column type is not always the same as the type used in your
schema. For example, a schema that has a `:string` field can be supported by
columns of type `:char`, `:varchar`, `:text`, and others. For this reason,
this function also accepts `:text` and other type annotations that are native
to the database. These are passed to the database as-is.

To sum up, the column type may be either an Ecto primitive type,
which is normalized in cases where the database does not understand it,
such as `:string` or `:binary`, or a database type which is passed as-is.
Custom Ecto types like `Ecto.UUID` are not supported because
they are application-level concerns and may not always map to the database.

Note: It may be necessary to quote case-sensitive, user-defined type names.
For example, PostgreSQL normalizes all identifiers to lower case unless
they are wrapped in double quotes. To ensure a case-sensitive type name
is sent properly, it must be defined `:'"LikeThis"'` or `:""LikeThis""`.
This is not necessary for column names because Ecto quotes them automatically.
Type names are not automatically quoted because they may be expressions such
as `varchar(255)`.

## Examples

    create table("posts") do
      add :title, :string, default: "Untitled"
    end

    alter table("posts") do
      add :summary, :text               # Database type
      add :object,  :map                # Elixir type which is handled by the database
      add :custom, :'"UserDefinedType"' # A case-sensitive, user-defined type name
      add :identity, :integer, generated: "BY DEFAULT AS IDENTITY" # Postgres generated identity column
      add :generated_psql, :string, generated: "ALWAYS AS (id::text) STORED" # Postgres calculated column
      add :generated_other, :string, generated: "CAST(id AS char)" # MySQL and TDS calculated column
    end

## Options

  * `:primary_key` - when `true`, marks this field as the primary key.
    If multiple fields are marked, a composite primary key will be created.
  * `:default` - the column's default value. It can be a string, number, empty
    list, list of strings, list of numbers, or a fragment generated by
    `fragment/1`.
  * `:null` - determines whether the column accepts null values. When not specified,
    the database will use its default behaviour (which is to treat the column as nullable
    in most databases).
  * `:size` - the size of the type (for example, the number of characters).
    The default is no size, except for `:string`, which defaults to `255`.
  * `:precision` - the precision for a numeric type. Required when `:scale` is
    specified.
  * `:scale` - the scale of a numeric type. Defaults to `0`.
  * `:comment` - adds a comment to the added column.
  * `:collation` - the collation of the text type.
  * `:after` - positions field after the specified one. Only supported on MySQL,
    it is ignored by other databases.
  * `:generated` - a string representing the expression for a generated column. See
    above for a comprehensive set of examples for each of the built-in adapters. If
    specified alongside `:start_value`/`:increment`, those options will be ignored.
  * `:start_value` - option for `:identity` key, represents initial value in sequence
    generation. Default is defined by the database.
  * `:increment` - option for `:identity` key, represents increment value for
    sequence generation. Default is defined by the database.
  * `:fields` - option for `:duration` type. Restricts the set of stored interval fields
    in the database.

# `add_if_not_exists`

Adds a column if it does not exist yet when altering a table.

If the `type` value is a `%Reference{}`, it is used to add a constraint.

`type` and `opts` are exactly the same as in `add/3`.

This command is not reversible as Ecto does not know about column existence before the creation attempt.

## Examples

    alter table("posts") do
      add_if_not_exists :title, :string, default: ""
    end

# `alter`
*macro* 

Alters a table.

## Examples

    alter table("posts") do
      add :summary, :text
      modify :title, :text
      remove :views
    end

# `constraint`

Defines a constraint (either a check constraint or an exclusion constraint)
to be evaluated by the database when a row is inserted or updated.

## Examples

    create constraint("users", :price_must_be_positive, check: "price > 0")
    create constraint("size_ranges", :no_overlap, exclude: ~s|gist (int4range("from", "to", '[]') WITH &&)|)
    drop   constraint("products", "price_must_be_positive")

## Options

  * `:check` - A check constraint expression. Required when creating a check constraint.
  * `:exclude` - An exclusion constraint expression. Required when creating an exclusion constraint.
  * `:prefix` - The prefix for the table.
  * `:validate` - Whether or not to validate the constraint on creation (true by default). See the section below for more information
  * `:comment` - adds a comment to the constraint.

## Using `validate: false`

Validation/Enforcement of a constraint is enabled by default, but disabling on constraint
creation is supported by PostgreSQL, and MySQL, and can be done by setting `validate: false`.

Setting `validate: false` as an option can be useful, as the creation of a constraint will cause
a full table scan to check existing rows. The constraint will still be enforced for subsequent
inserts and updates, but should then be updated in a following command or migration to enforce
the new constraint.

Validating / Enforcing the constraint in a later command, or migration, can be done like so:

```
  def change do
    # PostgreSQL
    execute "ALTER TABLE products VALIDATE CONSTRAINT price_must_be_positive", ""

    # MySQL
    execute "ALTER TABLE products ALTER CONSTRAINT price_must_be_positive ENFORCED", ""
  end
```

See the [Safe Ecto Migrations guide](safe_migrations.html) for an
in-depth explanation of the benefits of this approach.

# `create`

Creates one of the following:

  * an index
  * a table with only the :id primary key
  * a constraint

When reversing (in a `change/0` running backwards), indexes are only dropped
if they exist, and no errors are raised. To enforce dropping an index, use
`drop/1`.

## Examples

    create index("posts", [:name])
    create table("version")
    create constraint("products", "price_must_be_positive", check: "price > 0")

# `create`
*macro* 

Creates a table.

By default, the table will also include an `:id` primary key field that
has a type of `:bigserial`. Check the `table/2` docs for more information.

## Examples

    create table(:posts) do
      add :title, :string, default: "Untitled"
      add :body,  :text

      timestamps()
    end

# `create_if_not_exists`

Creates an index or a table with only `:id` field if one does not yet exist.

## Examples

    create_if_not_exists index("posts", [:name])

    create_if_not_exists table("version")

# `create_if_not_exists`
*macro* 

Creates a table if it does not exist.

Works just like `create/2` but does not raise an error when the table
already exists.

# `direction`

```elixir
@spec direction() :: :up | :down
```

Gets the migrator direction.

# `drop`

Drops one of the following:

  * an index
  * a table
  * a constraint

## Examples

    drop index("posts", [:name])
    drop table("posts")
    drop constraint("products", "price_must_be_positive")
    drop index("posts", [:name]), mode: :cascade
    drop table("posts"), mode: :cascade

## Options

  * `:mode` - when set to `:cascade`, automatically drop objects that depend
    on the index, and in turn all objects that depend on those objects
    on the table. Default is `:restrict`

# `drop_if_exists`

Drops one of the following if it exists:

  * an index
  * a table
  * a constraint

Does not raise an error if the specified table or index does not exist.

## Examples

    drop_if_exists index("posts", [:name])
    drop_if_exists table("posts")
    drop_if_exists constraint("products", "price_must_be_positive")
    drop_if_exists index("posts", [:name]), mode: :cascade
    drop_if_exists table("posts"), mode: :cascade

## Options

  * `:mode` - when set to `:cascade`, automatically drop objects that depend
    on the index, and in turn all objects that depend on those objects
    on the table. Default is `:restrict`

# `execute`

Executes arbitrary SQL, anonymous function or a keyword command.

The argument is typically a string, containing the SQL command to be executed.
Keyword commands exist for non-SQL adapters and are not used in most
situations.

You may instead run arbitrary code as part of your migration by supplying an
anonymous function. This defers execution of the anonymous function until
the migration callback has terminated (see [Executing and
flushing](#module-executing-and-flushing)). This is most often used in
combination with `repo/0` by library authors who want to create high-level
migration helpers.

Reversible commands can be defined by calling `execute/2`.

## Examples

    execute "CREATE EXTENSION postgres_fdw"

    execute create: "posts", capped: true, size: 1024

    execute(fn -> repo().query!("SELECT $1::integer + $2", [40, 2], [log: :info]) end)

    execute(fn -> repo().update_all("posts", set: [published: true]) end)

# `execute`

Executes reversible SQL commands.

This is useful for database-specific functionality that does not
warrant special support in Ecto, for example, creating and dropping
a PostgreSQL extension. The `execute/2` form avoids having to define
separate `up/0` and `down/0` blocks that each contain an `execute/1`
expression.

The allowed parameters are explained in `execute/1`.

## Examples

    defmodule MyApp.MyMigration do
      use Ecto.Migration

      def change do
        execute "CREATE EXTENSION postgres_fdw", "DROP EXTENSION postgres_fdw"
        execute(&execute_up/0, &execute_down/0)
      end

      defp execute_up, do: repo().query!("select 'Up query …';", [], [log: :info])
      defp execute_down, do: repo().query!("select 'Down query …';", [], [log: :info])
    end

# `execute_file`

Executes a SQL command from a file.

The argument must be a path to a file containing a SQL command.

Reversible commands can be defined by calling `execute_file/2`.

# `execute_file`

Executes reversible SQL commands from files.

Each argument must be a path to a file containing a SQL command.

See `execute/2` for more information on executing SQL commands.

# `flush`
*macro* 

Execute all changes specified by the migration so far.

See [Executing and flushing](#module-executing-and-flushing).

# `fragment`

Generates a fragment to be used as a default value.

## Examples

    create table("posts") do
      add :inserted_at, :naive_datetime, default: fragment("now()")
    end

# `index`

Returns an index struct that can be given to `create/1`, `drop/1`, etc.

Expects the table name as the first argument and the index field(s) as
the second. The fields can be atoms, representing columns, or strings,
representing expressions that are sent as-is to the database.

## Options

  * `:name` - the name of the index. Can be provided as a string or an atom.
  Defaults to "#{table}_#{column}_index".
  * `:prefix` - specify an optional prefix for the index.
  * `:unique` - indicates whether the index should be unique. Defaults to `false`.
  * `:comment` - adds a comment to the index.
  * `:using` - configures the index type.

Some options are supported only by some databases:

  * `:concurrently` - indicates whether the index should be created/dropped
    concurrently in MSSQL and PostgreSQL.
  * `:include` - specify fields for a covering index,
    [supported by PostgreSQL only](https://www.postgresql.org/docs/current/indexes-index-only-scans.html).
  * `:nulls_distinct` - specify whether null values should be considered
    distinct for a unique index. Defaults to `nil`, which will not add the
    parameter to the generated SQL and thus use the database default.
    This option is currently only supported by PostgreSQL 15+.
    For MySQL, it is always false. For MSSQL, it is always true.
    See the dedicated section on this option for more information.
  * `:only` - indicates to not recurse creating indexes on partitions.
    [supported by PostgreSQL only](https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE-MAINTENANCE).
  * `:options` - configures index options (WITH clause) for both PostgreSQL
    and MSSQL
  * `:where` - specify conditions for a partial index (PostgreSQL) /
    filtered index (MSSQL).

## Adding/dropping indexes concurrently

PostgreSQL supports adding/dropping indexes concurrently (see the
[docs](http://www.postgresql.org/docs/current/static/sql-createindex.html)).
However, this feature does not work well with the transactions used by
Ecto to guarantee integrity during migrations.

You can address this with two changes:

  1. Change your repository to use PG advisory locks as the migration lock.
     Note this may not be supported by Postgres-like databases and proxies.

  2. Disable DDL transactions. Doing this removes the guarantee that all of
    the changes in the migration will happen at once, so you will want to
    keep it short.

If the database adapter supports several migration lock strategies, such as
Postgrex, then review those strategies and consider using a strategy that
utilizes advisory locks to facilitate running migrations one at a time even
across multiple nodes. For example:

### Config file (PostgreSQL)

    config MyApp.Repo, migration_lock: :pg_advisory_lock

### Migration file

    defmodule MyRepo.Migrations.CreateIndexes do
      use Ecto.Migration
      @disable_ddl_transaction true

      def change do
        create index("posts", [:slug], concurrently: true)
      end
    end

Alternately, you can add `@disable_migration_lock true` to your migration file.
This would mean that different nodes in a multi-node setup could run the same
migration at once. It is recommended to isolate your migrations to a single node
when using concurrent index creation without an advisory lock.

## Index types

When creating an index, the index type can be specified with the `:using`
option. The `:using` option can be an atom or a string, and its value is
passed to the generated `USING` clause as-is.

For example, PostgreSQL supports several index types like B-tree (the
default), Hash, GIN, and GiST. More information on index types can be found
in the [PostgreSQL docs](http://www.postgresql.org/docs/current/indexes-types.html).

## Partial indexes

Databases like PostgreSQL and MSSQL support partial indexes.

A partial index is an index built over a subset of a table. The subset
is defined by a conditional expression using the `:where` option.
The `:where` option can be an atom or a string; its value is passed
to the generated `WHERE` clause as-is.

More information on partial indexes can be found in the [PostgreSQL
docs](http://www.postgresql.org/docs/current/indexes-partial.html).

## The `:nulls_distinct` option

A unique index does not prevent multiple null values by default in most databases.

For example, imagine we have a "products" table and need to guarantee that
sku's are unique within their category, but the category is optional.
Creating a regular unique index over the sku and category_id fields with:

    create index("products", [:sku, :category_id], unique: true)

will allow products with the same sku to be inserted if their category_id is `nil`.
The `:nulls_distinct` option can be used to change this behavior by considering
null values as equal, i.e. not distinct:

    create index("products", [:sku, :category_id], unique: true, nulls_distinct: false)

This option is currently only supported by PostgreSQL 15+.
As a workaround for older PostgreSQL versions and other databases, an
additional partial unique index for the sku can be created:

    create index("products", [:sku, :category_id], unique: true)
    create index("products", [:sku], unique: true, where: "category_id IS NULL")

## Sorting direction

You can specify the sorting direction of the index by using a keyword list:

    create index("products", [desc: sku])

The following keywords are supported:

  * `:asc`
  * `:asc_nulls_last`
  * `:asc_nulls_first`
  * `:desc`
  * `:desc_nulls_last`
  * `:desc_nulls_first`

The `*_nulls_first` and `*_nulls_last` variants are not supported by all
databases.

## Examples

    # With no name provided, the name of the below index defaults to
    # products_category_id_sku_index
    create index("products", [:category_id, :sku], unique: true)

    # The name can also be set explicitly
    create index("products", [:category_id, :sku], name: :my_special_name)

    # Indexes can be added concurrently
    create index("products", [:category_id, :sku], concurrently: true)

    # The index type can be specified
    create index("products", [:name], using: :hash)

    # Partial indexes are created by specifying a :where option
    create index("products", [:user_id], where: "price = 0", name: :free_products_index)

    # Covering indexes are created by specifying a :include option
    create index("products", [:user_id], include: [:category_id])

Indexes also support custom expressions. Some databases may require the
index expression to be written between parentheses:

    # Create an index on a custom expression
    create index("products", ["(lower(name))"], name: :products_lower_name_index)

    # Create a tsvector GIN index on PostgreSQL
    create index("products", ["(to_tsvector('english', name))"],
                 name: :products_name_vector, using: "GIN")

If the expression is a column name, it will not be quoted. This may cause issues
when the column is named after a reserved word. Consider using an atom instead.
For example, the name `offset` is reserved in many databases so the following
could produce an error: `create index("products", ["offset"])`.

# `modify`

Modifies the type of a column when altering a table.

This command is not reversible unless the `:from` option is provided.
When the `:from` option is set, the adapter will try to drop
the corresponding foreign key constraints before modifying the type.
Generally speaking, you want to pass the type and each option
you are modifying to `:from`, so the column can be rolled back properly.
However, note that `:from` cannot be used to modify primary keys,
as those are generally trickier to revert.

See `add/3` for more information on supported types.

If you want to modify a column without changing its type,
such as adding or dropping a null constraints, consider using
the `execute/2` command with the relevant SQL command instead
of `modify/3`, if supported by your database. This may avoid
redundant type updates and be more efficient, as an unnecessary
type update can lock the table, even if the type actually
doesn't change.

## Examples

    alter table("posts") do
      modify :title, :text
    end

    # Self rollback when using the :from option
    alter table("posts") do
      modify :title, :text, from: :string
    end

    # Modify column with rollback options
    alter table("posts") do
      modify :title, :text, null: false, from: {:string, null: true}
    end

    # Modify the :on_delete option of an existing foreign key
    alter table("comments") do
      modify :post_id, references(:posts, on_delete: :delete_all),
        from: references(:posts, on_delete: :nothing)
    end

## Options

  * `:null` - determines whether the column accepts null values. If this option is
    not set, the nullable behaviour of the underlying column is not modified.
  * `:default` - changes the default value of the column.
  * `:from` - specifies the current type and options of the column.
  * `:size` - specifies the size of the type (for example, the number of characters).
    The default is no size.
  * `:precision` - the precision for a numeric type. Required when `:scale` is
    specified.
  * `:scale` - the scale of a numeric type. Defaults to `0`.
  * `:comment` - adds a comment to the modified column.
  * `:collation` - the collation of the text type.

# `prefix`

Gets the migrator prefix.

# `references`

Defines a foreign key.

By default it assumes you are linking to the referenced table
via its primary key with name `:id`. If you are using a non-default
key setup (e.g. using `uuid` type keys) you must ensure you set the
options, such as `:column` and `:type`, to match your target key.

## Examples

    create table("products") do
      add :group_id, references("groups")
    end

    create table("categories") do
      add :group_id, :integer
      # A composite foreign that points from categories (product_id, group_id)
      # to products (id, group_id)
      add :product_id, references("products", with: [group_id: :group_id])
    end

## Options

  * `:name` - The name of the underlying reference, which defaults to
    "#{table}_#{column}_fkey".
  * `:column` - The column name in the referenced table, which defaults to `:id`.
  * `:prefix` - The prefix for the reference. Defaults to the prefix
    defined by the block's `table/2` struct (the "products" table in
    the example above), or `nil`.
  * `:type` - The foreign key type, which defaults to `:bigserial`.
  * `:on_delete` - What to do if the referenced entry is deleted. May be
    `:nothing` (default), `:delete_all`, `:nilify_all`, `{:nilify, columns}`, `:default_all`, `{:default, columns}`
    or `:restrict`. `{:nilify, columns}` and `{:default, columns}` expect a list of atoms for `columns`
    and is not supported by all databases.
  * `:on_update` - What to do if the referenced entry is updated. May be
    `:nothing` (default), `:update_all`, `:nilify_all`, or `:restrict`.
  * `:validate` - Whether or not to validate the foreign key constraint on
     creation or not. Only available in PostgreSQL, and should be followed by
     a command to validate the foreign key in a following migration if false.
  * `:with` - defines additional keys to the foreign key in order to build
    a composite foreign key
  * `:match` - select if the match is `:simple`, `:partial`, or `:full`. This is
    [supported only by PostgreSQL](https://www.postgresql.org/docs/current/sql-createtable.html)
    at the moment.

# `remove`

Removes a column when altering a table.

This command is not reversible as Ecto does not know what type it should add
the column back as. See `remove/3` as a reversible alternative.

## Examples

    alter table("posts") do
      remove :title
    end

# `remove`

Removes a column in a reversible way when altering a table.

`type` and `opts` are exactly the same as in `add/3`, and
they are used when the command is reversed.

If the `type` value is a `%Reference{}`, it is used to remove the constraint.

## Examples

    alter table("posts") do
      remove :title, :string, default: ""
    end

# `remove_if_exists`

Removes a column if the column exists.

This command is not reversible as Ecto does not know whether or not the column existed before the removal attempt.

## Examples

    alter table("posts") do
      remove_if_exists :title
    end

# `remove_if_exists`

Removes a column if the column exists.

If the type is a reference, removes the foreign key constraint for the reference first, if it exists.

This command is not reversible as Ecto does not know whether or not the column existed before the removal attempt.

## Examples

    alter table("posts") do
      remove_if_exists :author_id, references(:authors)
    end

# `rename`

Renames a table or index.

## Examples

    # rename a table
    rename table("posts"), to: table("new_posts")

    # rename an index
    rename(index(:people, [:name], name: "persons_name_index"), to: "people_name_index")

# `rename`

Renames a column.

Note that this occurs outside of the `alter` statement.

## Examples

    rename table("posts"), :title, to: :summary

# `repo`

```elixir
@spec repo() :: Ecto.Repo.t()
```

Gets the migrator repo.

# `table`

Returns a table struct that can be given to `create/2`, `alter/2`, `drop/1`,
etc.

## Examples

    create table("products") do
      add :name, :string
      add :price, :decimal
    end

    drop table("products")

    create table("products", primary_key: false) do
      add :name, :string
      add :price, :decimal
    end

    create table("daily_prices", primary_key: false, options: "PARTITION BY RANGE (date)") do
      add :name, :string, primary_key: true
      add :date, :date, primary_key: true
      add :price, :decimal
    end

    create table("users", primary_key: false) do
      add :id, :identity, primary_key: true, start_value: 100, increment: 20
    end

## Options

  * `:primary_key` - when `false`, a primary key field is not generated on table
    creation. Alternatively, a keyword list in the same style of the
    `:migration_primary_key` repository configuration can be supplied
    to control the generation of the primary key field. The keyword list
    must include `:name` and `:type`. See `add/3` for further options.
  * `:engine` - customizes the table storage for supported databases. For MySQL,
    the default is InnoDB.
  * `:prefix` - the prefix for the table. This prefix will automatically be used
    for all constraints and references defined for this table unless explicitly
    overridden in said constraints/references.
  * `:comment` - adds a comment to the table.
  * `:options` - provide custom options that will be appended after the generated
    statement. For example, "WITH", "INHERITS", or "ON COMMIT" clauses. "PARTITION BY"
    can be provided for databases that support table partitioning.
  * `:modifiers` - provide custom modifiers that should be inserted to the
    table creation statement, between the tokens "CREATE" and "TABLE". For
    example, "UNLOGGED", "GLOBAL", "TEMPORARY", or "GLOBAL TEMPORARY" in
    PostgreSQL.

# `timestamps`

Adds `:inserted_at` and `:updated_at` timestamp columns.

Those columns are of `:naive_datetime` type. A list of `opts` can be given
to customize the generated fields.

Following options will override the repo configuration specified by
`:migration_timestamps` option.

## Options

  * `:inserted_at` - the name of the column for storing insertion times.
    Setting it to `false` disables the column.
  * `:updated_at` - the name of the column for storing last-updated-at times.
    Setting it to `false` disables the column.
  * `:type` - the type of the `:inserted_at` and `:updated_at` columns.
    Defaults to `:naive_datetime`.
  * `:default` - the columns' default value. It can be a string, number, empty
    list, list of strings, list of numbers, or a fragment generated by
    `fragment/1`.
  * `:null` - determines whether the column accepts null values. Defaults to
    `false`.

## Timezones

When using `type: :utc_datetime` or `type: :utc_datetime_usec`, Ecto already
guarantees that all input data must be in UTC format, regardless of your
database's timezone. For this reason, Ecto defaults to `timestamp`, even in
PostgreSQL.

If you chose to use `type: :timestamptz`, then you must ensure that your
database is configured to "Etc/UTC", otherwise Ecto and PostgreSQL will be
out of sync. `mix ecto.create` will create databases in Etc/UTC from
Ecto v3.14.0, but it is still your responsibility to configure your production
database accordingly. If you use `:timestamptz` and do not configure the
timezone, it may lead to incorrect behaviour or Ecto refusing to load data.

# `unique_index`

Shortcut for creating a unique index.

See `index/3` for more information.

---

*Consult [api-reference.md](api-reference.md) for complete listing*