Bug #80206 EQ_REF extension should be skipped for the last joined table
Submitted: 30 Jan 2016 6:13
Reporter: Mikiya Okuno Email Updates:
Status: Verified Impact on me:
None 
Category:MySQL Server: Optimizer Severity:S3 (Non-critical)
Version:5.7.10 OS:Any
Assigned to: CPU Architecture:Any

[30 Jan 2016 6:13] Mikiya Okuno
Description:
If N tables are joined and N-1 th table is joined by eq_ref, the optimizer attempts to test if EQ_REF extension is possible for N th joined table. However, even if N th table is eq_ref and "added_to_eq_ref_extension", there is no remaining tables to prune. IMHO, Optimize_table_order::eq_ref_extension_by_limited_search() should be skipped if number of remaining joined tables is 1. Thus, if total number of joined tables, say N, is less than 4, Optimize_table_order::eq_ref_extension_by_limited_search() is not needed at all.

This will save some CPU cycles.

How to repeat:
n/a

Suggested fix:
n/a