问题
I want to run many select queries at once by putting them between BEGIN; END;. I tried the following:
cur = connection.cursor()
cur.execute("""
BEGIN;
SELECT ...;
END;""")
res = cur.fetchall()
However, I get the error:
psycopg2.ProgrammingError: no results to fetch
How can I actually get data this way?
Likewise, if I just have many selects in a row, I only get data back from the latest one. Is there a way to get data out of all of them?
回答1:
Postgresql doesn't actually support returning multiple result sets from a single command. If you pass this input to psql:
BEGIN;
SELECT ...;
END;
it will split this up client-side and actually execute three statements, only the second of which returns a result set.
"BEGIN" and "END" are SQL-level commands to start/finish a transaction. (There may be a lower-level protocol for doing this but I can't remember). You probably don't want to issue them directly, but rather have your driver (psycopg2) handle this. For example, with Perl's DBI I specify AutoCommit=>0 when connecting and it implicitly issues a "BEGIN" before my first command; and then "END" (or "COMMIT" etc) when I explicitly call $dbh->commit; I guess Python's DB-API works rather like this, since other systems such as JDBC do as well...
回答2:
If you're just SELECTing something and you don't have a function that performs any DML or the like, you shouldn't need to make an explicit transaction for any reason I'm aware of.
来源：https://stackoverflow.com/questions/3002033/sql-get-data-out-of-begin-end-block-in-python