Re: [HACKERS] Multicolumn hash indexes

On Wed, Sep 27, 2017 at 9:56 AM, Jesper Pedersen
<jesper.pedersen@redhat.com> wrote:
> Maybe an initial proof-of-concept could store the hash of the first column
> (col1) plus the hash of all columns (col1, col2, col3) in the index, and see
> what requirements / design decisions would appear from that.

I thought about that sort of thing yesterday but it's not that simple.
The problem is that the hash code isn't just stored; it's used to
assign tuples to buckets.  If you have two hash codes, you have to
pick one of the other to use for assigning the tuple to a bucket.  And
then if you want to search using the other hash code, you have to
search all of the buckets, which will stink.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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