Change in custom field search behaviour impacting on performance

On upgrading a site with a large data set to 4.6 we found that the searches on custom field data are now unacceptably slow - gone from 1 second to 1 minute.

The reason for this is a behaviour change somewhere since 4.5 (probably in 4.6 but I verified against 4.4 not 4.4 for previous behaviour).

I am specifically looking at a text field in advanced search.

My suspicion is that this change was an unintentional side-effect of code consolidation rather than an intentional change @monish.deb @colemanw @dgg any idea?

Note that the mysql rationale for the problem is that the index only works on the portion up until the first '%' so an '%' at the start creates a non-indexed filter.