问题
I have a relative simple query
SELECT
, db1.something
, COALESCE(db2.something_else, 'NA') AS something2
FROM dwh.db_1 AS db1
LEFT JOIN dwh.db_2 AS db2 ON db1.some_id = db2 = some_id
EXPLAIN gives an estimated time of something more than 15 seconds.
On the other hand, explain on the following, where we basically replaced the alias with the table name:
SELECT
, db1.something
, COALESCE(db_2.something_else, 'NA') AS something2
FROM dwh.db_1 AS db1
LEFT JOIN dwh.db_2 AS db2 ON db1.some_id = db2.some_id
gives an estimated time of over 4 hours, where it seems like the system is trying to execute a product join on some spool (I can't really follow the sequence of planning steps).
I always thought that aliases are just aliases and have no impact on perf.
回答1:
The estimated time is probably correct :-)
A Table-Alias is not really an alias, it replaces the tablename within that query. In Teradata using the original tablename doesn't result in an error message (as it does within most other DBMSes), but it causes a CROSS join.
Why? Well, Teradata was implemented before there was Standard SQL, the initial query language was called TEQUEL (TEradata QUEry Language), whose syntax didn't require to list tables within FROM. A simple RETRIEVE TableName.ColumnName carried enough information for the Parser/Optimizer to resolve tablename and columnname. There's no flag to switch it off, some client tools refuse to submit it, but you can still submit RETRIEVE in BTEQ.
Within that above example you're mixing old TEQUEL and SQL, there are 3 tables for the optimizer, but only one join-condition, this results in a CROSS join to the third table.
At least it's easy to spot in Explain. The optimizer will do this stupid join as last step, so scroll to the end and you will see joined using a product join, with a join condition of ("(1=1)").
来源：https://stackoverflow.com/questions/30353672/teradata-use-of-aliases-impacts-explain-estimation-of-time