Howie <caffeine@toodarkpark.org> writes:
> (1) why is it trying to do a cartesian join when not using the aliased
> tables?
That's what it's supposed to do. When you provide an alias for a table
name in FROM, then as far as the rest of that query is concerned, that
alias *is* the name of the table --- it has no other. When you refer
to the original table name in the WHERE clause, that's taken as creating
a separate table reference that's implicitly added to FROM. Your query
is a four-way join with only one join having a restriction clause :-(
The alias behavior is necessary in order to handle self-joins properly,
for example to find married couples: SELECT * FROM person, person other WHERE person.spouse = other.spouse;
This would be ambiguous if "person" were exposed by the second FROM clause.
SQL92 requires it to work this way:
<table reference> ::= <table name> [ [ AS ] <correlation name> [ <left paren>
<derivedcolumn list> <right paren> ] ]
...
1) A <correlation name> immediately contained in a <table refer- ence> TR is exposed by TR. A <table
name>immediately contained in a <table reference> TR is exposed by TR if and only if TR does not
specifya <correlation name>.
I think that implicitly adding a table to FROM is a Postgres extension
not found in SQL92 --- we probably really ought to reject such a query
with an error, since this behavior seems to be surprising...
regards, tom lane
