Why would you choose to execute "SELECT 1 \; SELECT 2;" instead of "SELECT 1; SELECT 2;" in a setup where the behavior of both strings is identical? Or, rather, how would they differ?
The answer is that at the protocol level the first one is one network round trip with the server, the second is two.
The difference probably does not really matter that much for psql which is more an interactive than a scripting tool.
However I'm really looking at that in the context of pgbench, which exhibits the same behavior.
If you run a bench and have one round trip or several over a local network, the latency is not the same. Consider a throttled read-only load composed of 3 random selects, several rounds (so.sql) vs one (so2.sql):
> pgbench -h server -T 100 -P 1 --rate 200 -c 2 -f so.sql -f so2.sql ... SQL script 1: so.sql - weight = 1 (targets 50.0% of total) - 10010 transactions (50.1% of total, tps = 100.101872) - latency average = 1.878 ms - latency stddev = 3.614 ms SQL script 2: so2.sql - weight = 1 (targets 50.0% of total) - 9954 transactions (49.9% of total, tps = 99.541861) - latency average = 1.089 ms - latency stddev = 3.022 msThere is 0.8 ms latency reduction, that is a 40% difference. Standard deviation is also significantly lower.
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