Allow upserting a pandas dataframe to a postgres table (equivalent to df.to_sql(..., if_exists='update')

upsert_df.py

# Upsert function for pandas to_sql with postgres
# https://stackoverflow.com/questions/1109061/insert-on-duplicate-update-in-postgresql/8702291#8702291
# https://www.postgresql.org/docs/devel/sql-insert.html#SQL-ON-CONFLICT
import pandas as pd
import sqlalchemy
import uuid
import os


def upsert_df(df: pd.DataFrame, table_name: str, engine: sqlalchemy.engine.Engine):
    """Implements the equivalent of pd.DataFrame.to_sql(..., if_exists='update')
    (which does not exist). Creates or updates the db records based on the
    dataframe records.
    Conflicts to determine update are based on the dataframes index.
    This will set primary keys on the table equal to the index names

    1. Create a temp table from the dataframe
    2. Insert/update from temp table into table_name

    Returns: True if successful

    """

    # If the table does not exist, we should just use to_sql to create it
    if not engine.execute(
        f"""SELECT EXISTS (
            SELECT FROM information_schema.tables 
            WHERE  table_schema = 'public'
            AND    table_name   = '{table_name}');
            """
    ).first()[0]:
        df.to_sql(table_name, engine)
        return True

    # If it already exists...
    temp_table_name = f"temp_{uuid.uuid4().hex[:6]}"
    df.to_sql(temp_table_name, engine, index=True)

    index = list(df.index.names)
    index_sql_txt = ", ".join([f'"{i}"' for i in index])
    columns = list(df.columns)
    headers = index + columns
    headers_sql_txt = ", ".join(
        [f'"{i}"' for i in headers]
    )  # index1, index2, ..., column 1, col2, ...

    # col1 = exluded.col1, col2=excluded.col2
    update_column_stmt = ", ".join([f'"{col}" = EXCLUDED."{col}"' for col in columns])

    # For the ON CONFLICT clause, postgres requires that the columns have unique constraint
    query_pk = f"""
    ALTER TABLE "{table_name}" ADD CONSTRAINT {table_name}_unique_constraint_for_upsert UNIQUE ({index_sql_txt});
    """
    try:
        engine.execute(query_pk)
    except Exception as e:
        # relation "unique_constraint_for_upsert" already exists
        if not 'unique_constraint_for_upsert" already exists' in e.args[0]:
            raise e

    # Compose and execute upsert query
    query_upsert = f"""
    INSERT INTO "{table_name}" ({headers_sql_txt}) 
    SELECT {headers_sql_txt} FROM "{temp_table_name}"
    ON CONFLICT ({index_sql_txt}) DO UPDATE 
    SET {update_column_stmt};
    """
    engine.execute(query_upsert)
    engine.execute(f'DROP TABLE "{temp_table_name}"')

    return True


if __name__ == "__main__":
    # TESTS (create environment variable DB_STR to do it)
    engine = sqlalchemy.create_engine(os.getenv("DB_STR"))

    indexes = ["id1", "id2"]

    df = pd.DataFrame(
        {
            "id1": [1, 2, 3, 3],
            "id2": ["a", "a", "b", "c"],
            "name": ["name1", "name2", "name3", "name4"],
            "age": [20, 32, 29, 68],
        }
    ).set_index(indexes)

    df_update = pd.DataFrame(
        {
            "id1": [1, 2, 3],
            "id2": ["a", "a", "b"],
            "name": ["surname1", "surname2", "surname3"],
            "age": [13, 44, 29],
        }
    ).set_index(indexes)

    df_insert = pd.DataFrame(
        {
            "id1": [1],
            "id2": ["d"],
            "name": ["dname"],
            "age": [100],
        }
    ).set_index(indexes)

    expected_result = (
        pd.DataFrame(
            {
                "id1": [1, 2, 3, 3, 1],
                "id2": ["a", "a", "b", "c", "d"],
                "name": ["surname1", "surname2", "surname3", "name4", "dname"],
                "age": [13, 44, 29, 68, 100],
            }
        )
        .set_index(indexes)
        .sort_index()
    )

    TNAME = "test_upsert_df"
    upsert_df(df=df, table_name=TNAME, engine=engine)
    upsert_df(df=df_update, table_name=TNAME, engine=engine)
    upsert_df(df=df_insert, table_name=TNAME, engine=engine)
    result = pd.read_sql_table(TNAME, engine).set_index(indexes).sort_index()
    assert (result == expected_result).all().all()
    print("Passed tests")

Hi Pascal, I never tried anything like it in SQLite. You can try providing a sqlite engine (instead of a postgres one) to the function to probe it.

Hey Pedro, do you know what causes the exception here? I started encountering it and I don't understand it. The table doesn't have the unique constraint but it thinks it does?

I think I know what causes this and how to fix this. I believe it is caused when two tables are using the same constraint name. It can be remedied with something like the following so that each table has a unique constraint name and they don't share them:

      constraint_name = f"unique_constraint_for_{table_name}_upsert"
      query_pk = f"""
      ALTER TABLE "{table_name}" DROP CONSTRAINT IF EXISTS {constraint_name};
      ALTER TABLE "{table_name}" ADD CONSTRAINT {constraint_name} UNIQUE ({index_sql_txt});
      """  # it's saying the index names must be unique
      engine.execute(query_pk)

I seemed to stop getting the error after making this change.

Yes that was the problem. Nice fix by the way, as far as I remember, I was creating and deleting the constraint in the end.

How does this compare to https://github.com/ThibTrip/pangres/wiki/Upsert ?

Edit: this solution produced an exception:

  File "/home/jupyter-admin/.local/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 717, in do_execute
    cursor.execute(statement, parameters)
psycopg2.errors.SyntaxError: syntax error at or near "-"

Resolved it by replacing with ThibTrip/pangres#74 (comment)