MySQL Bugs: #80206: EQ_REF extension should be skipped for the last joined table

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

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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