2017-06-11 18:34 GMT+02:00 Steven Grimm <sgrimm@xxxxxxxxxxxxxx>:
On Sun, Jun 11, 2017 at 8:21 AM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:Yeah, I've been watching this thread and trying to figure out how to
explain that part; I suspected a cause of this form but couldn't
make that theory match the 9-iterations observation. (I still can't.)I walked through the Java code in a debugger just now and have an explanation for the 5 vs. 9 discrepancy. The JDBC driver keeps a cache of queries that have been passed to a connection's prepareStatement() method, and inlines the bind values the first 4 times it sees a query in the hopes of reducing overhead on one-off queries. So I believe the sequence ends up being:1-4: JDBC driver inlines the values, server sees no bind variables
yes - this is client side prepared statement - prepareThreshold https://jdbc.postgresql.org/documentation/head/connect.html
Regards
Pavel
5: JDBC driver prepares the statement, server sees bind variables and tries generic plan6+: JDBC driver reuses the existing prepared statement from iteration 510: Server has seen the query 5 times before and switches to the custom planAs for the broader problem, at the risk of being hopelessly naive about how all this works internally: Could the discrepancy between the estimated and actual row counts be tracked and fed back into the planner somehow?-Steve
