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);
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.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
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