Backend touches index not involved in update / NULL indexing

Doug Mitchell (pgsql(at)dougmitchell(dot)com) reports a bug with a severity of 3
The lower the number the more severe it is.

Short Description
Backend touches index not involved in update / NULL indexing

Long Description
I had some updates that were taking a few seconds each on a primary-keyed table with a few million records. I dropped an index on a field not involved in the query and the updates now run several times faster.

Idea: The non-involved field which was indexed with a btree has a lot of NULLs in it. The btree node with the NULLs (several million records) had to be processed thus slowing performance. Please provide and document a way to turn on/off the indexing of NULL values.

Either way, PG should not have to update the index in the first place,
because it indexes a field which is not part of the SET or WHERE in the UPDATE statement.

Thanks,
Doug

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