Re: pgbench - allow to store select results into variables
От | Fabien COELHO |
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Тема | Re: pgbench - allow to store select results into variables |
Дата | |
Msg-id | alpine.DEB.2.20.1607140925400.1962@sto обсуждение исходный текст |
Ответ на | Re: pgbench - allow to store select results into variables (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
Hello Tom, >>> SELECT 1, 2 \; SELECT 3; >>> \into one two three > > Yeah, that's seriously nasty action-at-a-distance in my view. I'd be okay > with > > SELECT 1, 2 \into one two > SELECT 3 \into three ISTM that is not the same, because then you would have two queries (over the network) instead of one, so you pay the network latency twice? > but I do not think that a metacommand on a following line should > retroactively affect the execution of a prior command, much less commands > before the last one. Nope. The meta-command applies to the preceeding SQL command... which happens to be a \;-compound command. ISTM that all is logically fine. Some motivation about the feature (not its syntax or implementation), from a benchmarking perspective: - clients MUST read the server answers and possibly reuse them, hence a proposed \into feature. Discarding the answer as pgbench does not really comply with typical benchmark rules, eg from tpc-b: """1.3.2 Each transaction shall return to the driver the Account_Balance resulting from successful commit of the transaction. Comment: It is the intent of this clause that the account balance in the database be returned to the driver, i.e., thatthe application retrieve the account balance.""" - latency is important to applications (eg web applications), thus the ability to compound statements is a good thing. However, if in a bench one can compound statements but not retrieve their values, it fails the previous "retrieve the value" requirement. So basically I wish to avoid splitting compound queries and paying latency just because of a lack of syntax to do the right thing, hence the proposed feature which can retrieve data from various parts of a compound statement. -- Fabien.
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