Re: [PROPOSAL] Covering + unique indexes.

On 15/09/15 09:44, Jim Nasby wrote:
> On 9/14/15 1:50 PM, Thomas Munro wrote:
>> CREATE [UNIQUE {ON FIRST {COLUMN | n_unique_column COLUMNS}}
>> INDEX ON
>> table_name (column_name1, column_name2 ...);
>>
>>
>> I would use the first (simple) syntax and just throw an error
>> if the
>> user tries to skip a column on the UNIQUE clause.
>>
>> Seems, second option looks as more natural extension of CREATE
>> UNIQUE INDEX
>
> True, but it's awefully verbose. :( And...
>
>> It surprised me that you can INCLUDE extra columns on non-UNIQUE
>> indexes, since you could just add them as regular indexed columns for
>> the same effect. It looks like when you do that in SQL Server, the
>> extra columns are only stored on btree leaf pages and so can't be used
>> for searching or ordering. I don't know how useful that is or if we
>> would ever want it... but I just wanted to note that difference, and
>> that the proposed UNIQUE ON FIRST n COLUMNS syntax and catalog change
>> can't express that.
>
> ... we might want to support INCLUDE at some point. It enhances
> covering scans without bloating the heck out of the btree. (I'm not
> sure if it would help other index types...) So it seems like a bad
> idea to preclude that.
>
> I don't see that UNIQUE ON FIRST precludes also supporting INCLUDE.
> Presumably we could do either
>
> CREATE INDEX ... ON table (f1, f2, f3) UNIQUE(f1, f2) INCLUDE(f4);
Of the formats I've seen so far, I prefer this one.

I think using "[ALSO] INCLUDE(f4)" - might be potentially more readable
than using just "INCLUDE(f4)". even if not used, the noise word also
would help people understand that the other fields mentioned are already
covered.

If not too difficult then allowing the unique fields to be separated by
other fields could be useful - in the example allowing "UNIQUE(f1,
f3)". Especially if the index is likely to be used to CLUSTER a table,
where the order f1, f2, ... is important.

> or
> CREATE UNIQUE ON FIRST 2 COLUMNS INDEX ... ON table (f1, f2, f3)
> INCLUDE(f4);
>
> Personally, I find the first form easier to read.
>
> Are we certain that no index type could ever support an index on (f1,
> f2, f3) UNIQUE(f1, f3)? Even if it doesn't make sense for btree,
> perhaps some other index could handle it.