A Missing Link in Postgres 11: Fast Column Creation with Defaults

If you read through the release notes for upcoming Postgres 11, you might see a somewhat inconspicuous addition tucked away at the bottom of the enhancements list:

Many other useful performance improvements, including making ALTER TABLE .. ADD COLUMN with a non-null column default faster

It’s not a flagship feature of the new release, but it’s still one of the more important operational improvements that Postgres has made in years, even though it might not be immediately obvious why. The short version is that it’s eliminated a limitation that used to make correctness in schema design difficult, but let’s take a look at the details.

Alterations and exclusive locks

Consider for a moment one of the simplest database statements possible, one that adds a new column to a table:

ALTER TABLE users
    ADD COLUMN credits bigint;

Although it’s altering the table’s schema, any modern database is sophisticated enough to make this operation practically instantaneous. Instead of rewriting the existing representation of the table (thereby forcing all existing data to be copied over at great expense), information on the new column is added to the system catalog, which is cheap. That allows new rows to be written with values for the new column, and the system is smart enough to return NULL for current rows where no value previously existed.

But things get complicated when we add a DEFAULT clause to the same statement:

ALTER TABLE users
    ADD COLUMN credits bigint NOT NULL DEFAULT 0;

The SQL looks so similar as to be almost identical, but where the previous operation was trivial, this one is infinitely more expensive in that it now requires a full rewrite of the table and all its indexes. Because there’s now a non-null value involved, the database ensures data integrity by going back and injecting it into every existing row.

Despite that expense, Postgres is still capable of doing the rewrite efficiently, and on smaller databases it’ll appear to happen instantly.

It’s bigger installations where it becomes a problem. Rewriting a table with a large body of existing data will take about as long as you’d expect, and in the meantime, the rewrite will take an ACCESS EXCLUSIVE lock on the table. ACCESS EXCLUSIVE is the coarsest granularity of table lock possible, and it’ll block every other operation until it’s released; even simple SELECT statements have to wait. In any system with a lot of ongoing access to the table, that’s a huge problem.

Transactions blocking during a table rewrite.

Historically, accidentally locking access to a table when adding a column has been a common pitfall for new Postgres operators because there’s nothing in the SQL to tip them off to the additional expense of adding that DEFAULT clause. It takes a close reading of the manual to find out, or the pyrrhic wisdom acquired by causing a minor operational incident.

Constraints, relaxed by necessity

Because it’s not possible to cheaply add a DEFAULT column, it’s also not possible to add a column set to NOT NULL. By definition non-null columns need to have values for every row, and you can’t add one to a non-empty table without specifying what values the existing data should have, and that takes DEFAULT.

You can still get a non-null column by first adding it as nullable, running a migration to add values to every existing row, then altering the table with SET NOT NULL, but even that’s not perfectly safe because SET NOT NULL requires a full stable scan as it verifies the new constraint across all existing data. The scan is faster than a rewrite, but still needs an ACCESS EXCLUSIVE lock.

The amount of effort involved in getting a new non-null column into any large relation means that in practice you often don’t bother. It’s either too dangerous, or too time consuming.

Why bother with non-null anyway?

One of the biggest reasons to prefer relational databases over document stores, key/value stores, and other less sophisticated storage technology is data integrity. Columns are strongly typed with the likes of INT, DECIMAL, or TIMESTAMPTZ. Values are constrained with NOT NULL, VARCHAR (length), or CHECK constraints. Foreign key constraints guarantee referential integrity.

With a good schema design you can rest assured that your data is in a high quality state because the very database is ensuring it. This makes querying or changing it easier, and prevents an entire class of application-level bugs caused by data existing in an unexpected state. Enthusiasts like me have always argued in favor of strong data constraints, but knew also that new non-null fields often weren’t possible in Postgres when it was running at scale.

Postgres 11 brings in a change that makes ADD COLUMN with DEFAULT values fast by marshaling them for existing rows only as necessary. The expensive table rewrite and long hold on ACCESS EXCLUSIVE are eliminated, and a gaping hole in Postgres’ operational story is filled. It will now be possible to have both strong data integrity and strong operational guarantees.

Appendix: Under the hood

The change adds two new fields to pg_attribute, a system table that tracks information on every column in the database:

  • atthasmissing: Set to true when there are missing default values.
  • attmissingval: Contains the missing value.

As scans are returning rows, they check these new fields and return missing values where appropriate. New rows inserted into the table pick up the default values as they’re created so that there’s no need to check atthasmissing when returning their contents.

Fast column creation with existing rows loading defaults from pg_attribute.

The pg_attribute fields are only used as long as they have to be. If at any point the table is rewritten, Postgres takes the opportunity to insert the default value for every row and unset atthasmissing and attmissingval.

Due to the relative simplicity of attmissingval, this optimization only works for default values and function calls that are non-volatile 1. Using it with a volatile function like random() won’t set atthasmissing and adding the default will have to rewrite the table like it did before. Non-volatile function calls work fine though. For example, adding DEFAULT now() will put the transaction’s current value of now() into atthasmissing and all existing rows will inherit it, but any newly inserted rows will get a current value of now() as you’d expect.

There’s nothing all that difficult conceptually about this change, but its implementation wasn’t easy because the system is complex enough that there’s a lot of places where the new missing values have to be considered. See the patch that brought it in for full details.

1 Some functions in Postgres are explicitly marked as VOLATILE to indicate that their value can change within a single table scan like random() or timeofday(). Most functions are not volatile including many time/date functions like now().

Article
A Missing Link in Postgres 11: Fast Column Creation with Defaults

Published
August 28, 2018

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