_ddl.py

import re
from sqlalchemy.sql import compiler, sqltypes
import logging

logger = logging.getLogger(__name__)


class DatabricksIdentifierPreparer(compiler.IdentifierPreparer):
    """https://docs.databricks.com/en/sql/language-manual/sql-ref-identifiers.html"""

    legal_characters = re.compile(r"^[A-Z0-9_]+$", re.I)

    def __init__(self, dialect):
        super().__init__(dialect, initial_quote="`")


class DatabricksDDLCompiler(compiler.DDLCompiler):
    def post_create_table(self, table):
        post = [" USING DELTA"]
        if table.comment:
            comment = self.sql_compiler.render_literal_value(
                table.comment, sqltypes.String()
            )
            post.append("COMMENT " + comment)

        post.append("TBLPROPERTIES('delta.feature.allowColumnDefaults' = 'enabled')")
        return '\n'.join(post)

    def visit_unique_constraint(self, constraint, **kw):
        logger.warning("Databricks does not support unique constraints")
        pass

    def visit_check_constraint(self, constraint, **kw):
        logger.warning("This dialect does not support check constraints")
        pass

    def visit_identity_column(self, identity, **kw):
        """When configuring an Identity() with Databricks, only the always option is supported.
        All other options are ignored.

        Note: IDENTITY columns must always be defined as BIGINT. An exception will be raised if INT is used.

        https://www.databricks.com/blog/2022/08/08/identity-columns-to-generate-surrogate-keys-are-now-available-in-a-lakehouse-near-you.html
        """
        text = "GENERATED %s AS IDENTITY" % (
            "ALWAYS" if identity.always else "BY DEFAULT",
        )
        return text

    def visit_set_column_comment(self, create, **kw):
        return "ALTER TABLE %s ALTER COLUMN %s COMMENT %s" % (
            self.preparer.format_table(create.element.table),
            self.preparer.format_column(create.element),
            self.sql_compiler.render_literal_value(
                create.element.comment, sqltypes.String()
            ),
        )

    def visit_drop_column_comment(self, create, **kw):
        return "ALTER TABLE %s ALTER COLUMN %s COMMENT ''" % (
            self.preparer.format_table(create.element.table),
            self.preparer.format_column(create.element),
        )

    def get_column_specification(self, column, **kwargs):
        """
        Emit a log message if a user attempts to set autoincrement=True on a column.
        See comments in test_suite.py. We may implement implicit IDENTITY using this
        feature in the future, similar to the Microsoft SQL Server dialect.
        """
        if column is column.table._autoincrement_column or column.autoincrement is True:
            logger.warning(
                "Databricks dialect ignores SQLAlchemy's autoincrement semantics. Use explicit Identity() instead."
            )

        colspec = super().get_column_specification(column, **kwargs)
        if column.comment is not None:
            literal = self.sql_compiler.render_literal_value(
                column.comment, sqltypes.STRINGTYPE
            )
            colspec += " COMMENT " + literal

        return colspec


class DatabricksStatementCompiler(compiler.SQLCompiler):
    def limit_clause(self, select, **kw):
        """Identical to the default implementation of SQLCompiler.limit_clause except it writes LIMIT ALL instead of LIMIT -1,
        since Databricks SQL doesn't support the latter.

        https://docs.databricks.com/en/sql/language-manual/sql-ref-syntax-qry-select-limit.html
        """
        text = ""
        if select._limit_clause is not None:
            text += "\n LIMIT " + self.process(select._limit_clause, **kw)
        if select._offset_clause is not None:
            if select._limit_clause is None:
                text += "\n LIMIT ALL"
            text += " OFFSET " + self.process(select._offset_clause, **kw)
        return text