Thread: Avoid displaying unnecessary "Recheck Cond" in EXPLAIN ANALYZE output if the bitmap is non-lossy

Hi,

As specified in $subject, if the bitmap constructed by bitmap index
scan is non-lossy i.e. row-level bitmap, then showing "Recheck Cond"
in EXPLAIN ANALYZE output is pointless. However in EXPLAIN without
ANALYZE we can't say the bitmap is actually a non-lossy one, as we
don't actually construct the "original" bitmap, so showing "Recheck
Cond" in this case makes sense.

Attaching a small patch that corrects EXPLAIN ANALYZE output for bitmap scans.

Note: $subject is identified in [1].

Thoughts?

[1] - https://www.youtube.com/watch?v=UXKYAZOWDgk  ---> at 13:50 (mm:ss)

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> writes:
> As specified in $subject, if the bitmap constructed by bitmap index
> scan is non-lossy i.e. row-level bitmap, then showing "Recheck Cond"
> in EXPLAIN ANALYZE output is pointless. However in EXPLAIN without
> ANALYZE we can't say the bitmap is actually a non-lossy one, as we
> don't actually construct the "original" bitmap, so showing "Recheck
> Cond" in this case makes sense.

I do not think this change makes even a little bit of sense.
The recheck condition is part of the plan structure, it is not
execution statistics.

I compare this proposal to having EXPLAIN suppress plan tree nodes
entirely if they weren't executed.  We don't do that and it
wouldn't be an improvement.  Especially not for non-text output
formats, where the schema of fields that are presented ought to
be fixed for any given plan tree.

            regards, tom lane