Re: [WIP PATCH] Index scan offset optimisation using visibility map

Hello.

> 1. Charts are measured in percents of pgbench TPS, right?
Yes, correct. Actual values are calculated as TPS_of_patched /
TPS_of_vanilla. TPS was measured using single postgres process (one core)
(I was also did tests with multiple processes, but they shows pretty same
results).

> 2. For example, is 97% actually 3% degrade?
Yes, such degrade happens for indexes with high correlation and predicates
with low selectivity. In such cases 2-4% overhead is caused by index data
read and page visibility check. But it is possible to detect such cases in
planner and use regular IndexScan instead.

> 3. The results are obtained on actual "sort of TPC-B" script?
You could check testing script (
https://gist.github.com/michail-nikolaev/23e1520a1db1a09ff2b48d78f0cde91d)
for SQL queries.

But briefly:
* Vanilla pg_bench initialization
* ALTER TABLE pgbench_accounts drop constraint pgbench_accounts_pkey; --
drop non-required constraint
* UPDATE pgbench_accounts SET bid = TRUNC(RANDOM() * {ROWS_N} + 1 --
randomize BID (used for selectivy predicate)
* UPDATE pgbench_accounts SET aid = TRUNC(RANDOM() * {ROWS_N} + 1) WHERE
random() <= (1.0 - {CORRELATION}) -- emulate index correlation by changing
some part of AID values
* CREATE index test_index ON pgbench_accounts USING btree(aid, bid) --
create index used for test
* VACUUM FULL;
* VACUUM ANALYZE pgbench_accounts;
* SELECT * FROM pgbench_accounts WHERE aid > {RANDOM} and bid % 100 <=
{SELECTIVITY} order by aid limit 50

Thanks,
Michail.