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fix documentation for adding field with default #253

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ccba575
fix documentation for adding field with default
chdsbd Aug 25, 2022
040ae50
fix
chdsbd Aug 25, 2022
825b804
wip
chdsbd Jan 24, 2023
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20 changes: 11 additions & 9 deletions docs/docs/adding-field-with-default.md
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Original file line number Diff line number Diff line change
Expand Up @@ -18,28 +18,30 @@ This statement is only safe when your default is non-volatile.
:::

```sql
ALTER TABLE "core_recipe" ADD COLUMN "foo" integer DEFAULT 10 NOT NULL;
-- blocks reads and writes while schema is changed (fast)
ALTER TABLE "account" ADD COLUMN "foo" integer DEFAULT 10;
```

### adding a volatile default

Add the field as nullable, then set a default, backfill, and remove nullabilty.
Add the field without a default, set the default, and then backfill existing rows in batches.

Instead of:

```sql
ALTER TABLE "core_recipe" ADD COLUMN "foo" integer DEFAULT 10 NOT NULL;
-- blocks reads and writes while table is rewritten (slow)
ALTER TABLE "account" ADD COLUMN "ab_group" integer DEFAULT random();
```

Use:

```sql
ALTER TABLE "core_recipe" ADD COLUMN "foo" integer;
ALTER TABLE "core_recipe" ALTER COLUMN "foo" SET DEFAULT 10;
-- backfill column in batches
ALTER TABLE "core_recipe" ALTER COLUMN "foo" SET NOT NULL;
```
-- blocks reads and writes while schema is changed (fast)
ALTER TABLE "account" ADD COLUMN "ab_group" integer;
-- blocks reads and writes while schema is changed (fast)
ALTER TABLE "account" ALTER COLUMN "ab_group" SET DEFAULT random();

We add our column as nullable, set a default for new rows, backfill our column (ideally done in batches to limit locking), and finally remove nullability.
-- backfill existing rows in batches to set the "last_modified" column
```

See ["How not valid constraints work"](constraint-missing-not-valid.md#how-not-valid-validate-works) for more information on adding constraints as `NOT VALID`.
1 change: 1 addition & 0 deletions linter/src/rules/adding_field_with_default.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -106,6 +106,7 @@ ALTER TABLE "core_recipe" ADD COLUMN "foo" integer DEFAULT uuid();
let ok_sql = r#"
-- NON-VOLATILE
ALTER TABLE "core_recipe" ADD COLUMN "foo" integer DEFAULT 10;
ALTER TABLE "account" ADD COLUMN "last_modified" timestamptz DEFAULT now();
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@chdsbd chdsbd Aug 25, 2022

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our code for checking volatility of functions doesn't work. now() is stable, while a function like random is volatile.

select
  proname,
  pronamespace::regnamespace,
  provolatile
from pg_proc
where proname = 'now'
  or proname = 'random';

I think we could hard code in a list of functions and then warn when we can't know for certain with a more detailed error message.

"#;

let pg_version_11 = Some(Version::from_str("11.0.0").unwrap());
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