System Variables#

What is a System Variable?#

System variables, called configuration parameters in Postgres, are server-side key-value pairs that control the server’s behavior. You set a variable in your session with SET <name> TO <value>, and you read one with SHOW <name> or the current_setting() function. Server-wide defaults come from server configuration.

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 read individually in the Postgres shell with SHOW <name> or SELECT current_setting('<name>').

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 TO 10;
ERROR:  parameter "max_connections" cannot be changed now

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

Show System Variables#

Individual system variables can be read with SHOW <name>. SHOW ALL is not yet supported.