"""create books table

Revision ID: 1234567890ab
Revises: 
Create Date: 2023-05-25 10:00:00.000000

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects.postgresql import UUID


# revision identifiers, used by Alembic
revision = '1234567890ab'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    op.create_table(
        'books',
        sa.Column('id', UUID(as_uuid=True), primary_key=True, server_default=sa.text("uuid_generate_v4()")),
        sa.Column('title', sa.String, nullable=False, index=True),
        sa.Column('author', sa.String, nullable=False, index=True),
        sa.Column('description', sa.String, nullable=True),
        sa.Column('author_id', UUID(as_uuid=True), nullable=False),
        sa.Column('created_at', sa.TIMESTAMP(timezone=True), nullable=False, server_default=sa.func.now()),
        sa.Column('updated_at', sa.TIMESTAMP(timezone=True), nullable=False, server_default=sa.func.now(), onupdate=sa.func.now()),
        sa.ForeignKeyConstraint(['author_id'], ['authors.id'], ondelete='CASCADE'),
    )
    op.create_index('idx_books_title_author', 'books', ['title', 'author'], unique=True)


def downgrade():
    op.drop_index('idx_books_title_author', 'books')
    op.drop_table('books')

```

This Alembic migration script creates a new table called `books` with the following columns:

- `id` (UUID, primary key, generated using `uuid_generate_v4()`)
- `title` (String, not nullable, indexed)
- `author` (String, not nullable, indexed)
- `description` (String, nullable)
- `author_id` (UUID, not nullable, foreign key referencing `authors.id` table with `ON DELETE CASCADE`)
- `created_at` (TIMESTAMP with timezone, not nullable, default to current time)
- `updated_at` (TIMESTAMP with timezone, not nullable, default to current time, updated on each row update)

It also creates a unique index on the `title` and `author` columns combined.

The `downgrade()` function removes the index and drops the `books` table.