System Variables

What is a System Variable?

System variables are server-side key-value pairs. These variables have lifecycles between server restarts (PERSIST), between sessions within a single server lifetime (GLOBAL), and within a single client session (SESSION). Variables for narrowing scopes are initialized hierarchically: PERSIST -> GLOBAL -> SESSION.

How to use System Variables

System variables are most often managed through the SQL shell, although server-startup defaults can be manually set and persisted between server restarts.

Difference between Postgres and Doltgres System Variables

Doltgres only supports a subset of Postgres's system variables at the moment. The ones we do support should have the same lifecycle behavior as Postgres.

We also have Doltgres-specific system variables, which can be found here. Most dolt specific variables are prefixed with either dolt_... or the database's name (ex: mydb_...). These can be listed in the MySQL shell with show queries: show all.

Interaction with Doltgres Version Control

System variables are maintained outside of version control. Different clones of the same database can have different system variables.

Some system variables impact transaction, merge, and conflict resolution behavior. For example, @@dolt_force_transaction_commit both creates a new Doltgres commit for every SQL transaction, and dismisses merge conflicts in the process of auto-executing these commits.

A full list of Doltgres system variables and descriptions can be found here.

Example

Reading System Variables

show max_connections;
 max_connections
-----------------
 100
(1 row)

Writing System Variables

-- some variables are read only
SET max_connections 10;
Error 1105: Variable 'max_connections' is a read only variable

-- some variables are "dynamic" at session time
SET TIME ZONE = 'PST8PDT';

Show System Variables

The SHOW ALL shows all system variables, but this syntax is not yet supported.

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