How to fix Liquibase: Change set already exists in PostgreSQL

Connect to your PostgreSQL database and check if the changeset is already recorded:

```sql
SELECT id, author, filename, exectype, orderexecuted
FROM databasechangelog
WHERE id = 'your_changeset_id' AND author = 'your_author'
ORDER BY orderexecuted DESC;

Add preconditions to your changesets to prevent re-execution of idempotent operations. This is the best practice for complex migrations.

Example XML changelog:

<changeSet id="001" author="myauthor">
    <preConditions onFail="MARK_RAN">
        <not>
            <tableExists tableName="my_table" />
        </not>
    </preConditions>
    <createTable tableName="my_table">
        <column name="id" type="INT" />
    </createTable>
</changeSet>

The onFail="MARK_RAN" tells Liquibase to skip the changeset if the precondition fails, preventing the error.

If a changeset was modified after deployment and you want Liquibase to accept the new version, add the valid checksum:

liquibase calculate-checksum --changelog-file=db/changelog.xml --changeset-identifier=001::myauthor

Then add this checksum to your changeset:

<changeSet id="001" author="myauthor">
    <validCheckSum>9:c4a3293f78f30058f9144aeb8186beb7</validCheckSum>
    <!-- Rest of your changeset -->
</changeSet>

If your changeset uses property substitution that changes between environments, set the runOnChange attribute:

<changeSet id="001" author="myauthor" runOnChange="true">
    <sql>
        UPDATE my_table SET config_value = '${env.config}'
    </sql>
</changeSet>

Alternatively, use runAlways="true" if the changeset should execute every time Liquibase runs.

Search your changelog files for duplicate IDs:

grep -r 'id="' db/changelog/ | sort | uniq -d

For each duplicate, rename one of them to have a unique ID:

<!-- Before -->
<changeSet id="001" author="myauthor">
    <!-- changes -->
</changeSet>

<!-- After -->
<changeSet id="001-renamed" author="myauthor">
    <!-- changes -->
</changeSet>

When merging branches with conflicting changelog files:

1. Don't modify existing changeset IDs
2. Append new changesets instead of editing old ones
3. Use a numbering scheme that prevents collisions (e.g., timestamps: id="202501281000")
4. Verify no duplicate IDs exist before committing:

grep -oh 'id="[^"]*"' db/changelog/*.xml | sort | uniq -c | grep -v '^ *1 '

If you upgraded the PostgreSQL JDBC driver and now get 'relation already exists' errors:

1. Check your JDBC driver version:
- Current version: 42.7.5+ (check pom.xml or build.gradle)

2. Verify uppercase database names in connection string:

jdbc:postgresql://localhost:5432/MYDB  // Problematic
   jdbc:postgresql://localhost:5432/mydb  // Correct

3. Either:
- Use lowercase database names, OR
- Downgrade to PostgreSQL JDBC 42.7.4 temporarily, OR
- Wait for a Liquibase/driver patch

4. Test with:

liquibase status --db-url=jdbc:postgresql://localhost:5432/mydb

If you're on Liquibase 4.25.1 or later and have duplicate changesets that can't be merged immediately:

liquibase update --allow-duplicated-changeset-identifiers=true

This allows Liquibase to deploy multiple changesets with the same ID, but only creates one database changelog entry. Use this as a temporary measure while you work to make IDs unique.