pgschema picks migration strategies that minimize downtime and blocking when making schema changes to production PostgreSQL databases.

​
Index Operations

​
Adding Indexes

When adding new indexes, pgschema automatically uses CREATE INDEX CONCURRENTLY to avoid blocking table writes: 
-- Generated migration
CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_email_status ON users (email, status DESC);

-- pgschema:wait
SELECT 
    COALESCE(i.indisvalid, false) as done,
    CASE 
        WHEN p.blocks_total > 0 THEN p.blocks_done * 100 / p.blocks_total
        ELSE 0
    END as progress
FROM pg_class c
LEFT JOIN pg_index i ON c.oid = i.indexrelid
LEFT JOIN pg_stat_progress_create_index p ON c.oid = p.index_relid
WHERE c.relname = 'idx_users_email_status';
The pgschema:wait directive blocks the schema migration execution, polls the database to monitor progress, displays the progress to the user, and automatically continues when the operation completes. It tracks:
  • Completion status: Whether the index is valid and ready
  • Progress percentage: Based on pg_stat_progress_create_index

​
Modifying Indexes

When changing existing indexes, pgschema uses a safe “create new, drop old” pattern: 
-- 1. Create new index with temporary name
CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_email_pgschema_new ON users (email, status);

-- 2. Wait for completion with progress monitoring
-- pgschema:wait
SELECT 
    COALESCE(i.indisvalid, false) as done,
    CASE 
        WHEN p.blocks_total > 0 THEN p.blocks_done * 100 / p.blocks_total
        ELSE 0
    END as progress
FROM pg_class c
LEFT JOIN pg_index i ON c.oid = i.indexrelid
LEFT JOIN pg_stat_progress_create_index p ON c.oid = p.index_relid
WHERE c.relname = 'idx_users_email_pgschema_new';

-- 3. Drop old index (fast operation)
DROP INDEX idx_users_email;

-- 4. Rename new index to final name
ALTER INDEX idx_users_email_pgschema_new RENAME TO idx_users_email;
This approach ensures:
  • No downtime during index creation
  • Query performance is maintained throughout the migration
  • Rollback is possible until the old index is dropped

​
Constraint Operations

Supported constraint types:
  • Check constraints: Data validation rules
  • Foreign key constraints: Referential integrity with full options
  • NOT NULL constraints: Column nullability (via check constraint pattern)

​
CHECK Constraints

Constraints are added using PostgreSQL’s NOT VALID pattern: 
-- 1. Add constraint without validation (fast)
ALTER TABLE orders
ADD CONSTRAINT check_amount_positive CHECK (amount > 0) NOT VALID;

-- 2. Validate existing data (may take time, but non-blocking)
ALTER TABLE orders VALIDATE CONSTRAINT check_amount_positive;

​
Foreign Key Constraints

Foreign keys use the same NOT VALID pattern: 
ALTER TABLE employees
ADD CONSTRAINT employees_company_fkey 
FOREIGN KEY (tenant_id, company_id) 
REFERENCES companies (tenant_id, company_id) 
ON UPDATE CASCADE ON DELETE RESTRICT DEFERRABLE NOT VALID;

ALTER TABLE employees VALIDATE CONSTRAINT employees_company_fkey;

​
NOT NULL Constraints

Adding NOT NULL constraints uses a three-step process: 
-- 1. Add check constraint (non-blocking)
ALTER TABLE users ADD CONSTRAINT email_not_null CHECK (email IS NOT NULL) NOT VALID;

-- 2. Validate constraint (ensures data compliance)
ALTER TABLE users VALIDATE CONSTRAINT email_not_null;

-- 3. Set NOT NULL (fast, since data is already validated)
ALTER TABLE users ALTER COLUMN email SET NOT NULL;

​
Examples

​
Adding a Multi-Column Index

Schema change: 
-- old.sql
CREATE TABLE users (id int, email text, status text);

-- new.sql  
CREATE TABLE users (id int, email text, status text);
CREATE INDEX idx_users_email_status ON users (email, status DESC);
Generated migration: 
CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_email_status ON users (email, status DESC);

-- pgschema:wait
SELECT 
    COALESCE(i.indisvalid, false) as done,
    CASE 
        WHEN p.blocks_total > 0 THEN p.blocks_done * 100 / p.blocks_total
        ELSE 0
    END as progress
FROM pg_class c
LEFT JOIN pg_index i ON c.oid = i.indexrelid
LEFT JOIN pg_stat_progress_create_index p ON c.oid = p.index_relid
WHERE c.relname = 'idx_users_email_status';

​
Adding a Foreign Key Constraint

Schema change: 
-- old.sql
CREATE TABLE orders (id int, customer_id int);

-- new.sql
CREATE TABLE orders (
  id int, 
  customer_id int,
  CONSTRAINT orders_customer_fkey FOREIGN KEY (customer_id) REFERENCES customers(id)
);
Generated migration: 
ALTER TABLE orders
ADD CONSTRAINT orders_customer_fkey 
FOREIGN KEY (customer_id) REFERENCES customers(id) NOT VALID;

ALTER TABLE orders VALIDATE CONSTRAINT orders_customer_fkey;
For more examples, see the test cases in testdata/diff/online which demonstrate online DDL patterns for various PostgreSQL schema operations.