What does “Recheck Cond” in Explain result mean?

Question

From an example in PostgreSQL document:

EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 100 AND stringu1 = 'xxx';
                                  QUERY PLAN
------------------------------------------------------------------------------
 Bitmap Heap Scan on tenk1  (cost=5.04..229.43 rows=1 width=244)
   Recheck Cond: (unique1 < 100)
   Filter: (stringu1 = 'xxx'::name)
   ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..5.04 rows=101
 width=0)
         Index Cond: (unique1 < 100)

Am I correct that

first perform Bitmap Index Scan on all the rows for the first condition, and
then on the returned rows, perform Bitmap Heap Scan for the second condition?

Since Bitmap Index Scan already checks the Index Cond on unique1 < 100, why is there "Recheck Cond" on the same condition again in Bitmp heap Scan? What does "Recheck Cond" mean?

I am not sure I understand this related post https://dba.stackexchange.com/questions/106264/recheck-cond-line-in-query-plans-with-a-bitmap-index-scan

Thanks.

a_horse_with_no_name a_horse_with_no_name · Accepted Answer · 2018-06-21T05:58:53

This was explained by Tom Lane on the mailing list:

what is "Recheck condition" and why is it needed?

If the bitmap gets too large we convert it to "lossy" style, in which we only remember which pages contain matching tuples instead of remembering each tuple individually. When that happens, the table-visiting phase has to examine each tuple on the page and recheck the scan condition to see which tuples to return.

What does “Recheck Cond” in Explain result mean?

2 Answers