在stack overflow上看到的大牛:
from sqlalchemy import types,create_engine
dtyp = {c:types.VARCHAR(df[c].str.len().max())
for c in df.columns[df.dtypes == 'object'].tolist()}
df.to_sql(..., dtype=dtyp)
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https://stackoverflow.com/questions/42727990/speed-up-to-sql-when-writing-pandas-dataframe-to-oracle-database-using-sqlalch/42769557#42769557