Re: Sequential Scans On Complex Query With UNION - see why this fails

Michael Glaesemann <grzm(at)myrealbox(dot)com> writes:
> On Jan 12, 2006, at 12:21 , Tom Lane wrote:
>> I think what's happening is that the parser implicitly parenthesizes
>> like this:
>>
>> from ((a join b on a.x=b.y) join c on b.y=c.z)

> Any idea off hand if the SQL spec has anything to say on the subject?

SQL92 has this BNF:

<from clause> ::= FROM <table reference> [ { <comma> <table reference> }... ]

<table reference> ::=
<table name> [ [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ] ]
| <derived table> [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ]
| <joined table>

<derived table> ::= <table subquery>

<derived column list> ::= <column name list>

<joined table> ::=
<cross join>
| <qualified join>
| <left paren> <joined table> <right paren>

<cross join> ::=
<table reference> CROSS JOIN <table reference>

<qualified join> ::=
<table reference> [ NATURAL ] [ <join type> ] JOIN
<table reference> [ <join specification> ]

<join specification> ::=
<join condition>
| <named columns join>

<join condition> ::= ON <search condition>

<named columns join> ::=
USING <left paren> <join column list> <right paren>

<join type> ::=
INNER
| <outer join type> [ OUTER ]
| UNION

<outer join type> ::=
LEFT
| RIGHT
| FULL

What we're talking about is the <qualified join> production, whose
inputs are <table reference>s, and a <table reference> can be another
<qualified join> with or without surrounding parentheses. So AFAICS
SQL92 specifically allows both of these constructions.

regards, tom lane