Merge pull request #2062 from dolthub/fulghum/minor-doc-updates

Adding docs for the schema overriding system var

Author: fulghum

content/reference/sql/version-control/dolt-sysvars.md

@@ -10,6 +10,7 @@ title: Dolt System Variables
   - [dolt_allow_commit_conflicts](#dolt_allow_commit_conflicts)
   - [dolt_force_transaction_commit](#dolt_force_transaction_commit)
   - [dolt_log_level](#dolt_log_level)
+  - [dolt_override_schema](#dolt_override_schema)
   - [dolt_show_branch_databases](#dolt_show_branch_databases)
   - [dolt_show_system_tables](#dolt_show_system_tables)
   - [dolt_transaction_commit](#dolt_transaction_commit)
@@ -90,6 +91,40 @@ fresh> show databases;
 When set to `1`, this system variable causes all system tables to be show in `show tables` and in `information_schema.tables`. 
 Defaults to `0`.
 
+
+## `dolt_override_schema`
+
+When set to a commit hash, branch name, or tag name, Dolt will map all table data to the schema at the specified commit, 
+branch, or tag. This is useful when you have a query that runs with a specific schema, and you want to run it with
+data that has a different schema. For example, if you add a `Birthdate` column to the `People` table in the most recent commits
+in your database, you cannot reference that column in queries run against older commits. If you enable schema overriding, and
+set `@@dolt_override_schema` to a commit that contains the `Birthdate` column, you can run the same query with recent
+commits and with older commits, without having to modify the query for the schema changes in the older commits. Dolt will
+map the table data to the schema at the specified commit, branch, or tag, and fill in the missing columns with `NULL` values.
+
+```sql
+-- check out an older branch that has a different schema
+CALL dolt_checkout('olderBranch');
+
+-- running a query that references the Birthdate column will fail
+SELECT Name, Birthdate FROM People;
+column "Birthdate" could not be found in any table in scope
+
+-- turning on schema overriding allows us to automatically map our data to the schema at the specified commit
+SET @@dolt_override_schema = 'main';
+SELECT Name, Birthdate FROM People;
++-----------+-----------+
+| Name      | Birthdate |
++-----------+-----------+
+| Billy     | NULL      |
+| Jimbo     | NULL      |
++-----------+-----------+
+```
+
+Note that when this session variable is set, the active Dolt session becomes read-only. To disable schema overriding,
+simply set this variable to `NULL`.
+
+
 ## `dolt_transaction_commit`
 
 When set to `1`, this system variable creates a Dolt commit for every