On Wed, May 10, 2023 at 7:24 PM Peter J. Holzer <hjp-pg...@hjp.at> wrote:
> On 2023-05-10 16:35:04 +0200, Marc Millas wrote: > > Unique (cost=72377463163.02..201012533981.80 rows=1021522829864 > width=97) > > -> Gather Merge (cost=72377463163.02..195904919832.48 > rows=1021522829864 width=97) > ... > > -> Parallel Hash Left Join > (cost=604502.76..1276224253.51 rows=204304565973 width=97) > > Hash Cond: ((t1.col_ano)::text = (t2.col_ano)::text) > ... > > > > //so.. the planner guess that those 2 join will generate 1000 billions > rows... > > Are some of the col_ano values very frequent? If say the value 42 occurs > 1 million times in both table_a and table_b, the join will create 1 > trillion rows for that value alone. That doesn't explain the crash or the > disk usage, but it would explain the crazy cost (and would probably be a > hint that this query is unlikely to finish in any reasonable time). > > hp > > good guess, even if a bit surprising: there is one (and only one) "value" which fit your supposition: NULL 750000 in each table which perfectly fit the planner rows estimate. One question: what is postgres doing when it planned to hash 1000 billions rows ? Did postgres create an appropriate ""space"" to handle those 1000 billions hash values ? thanks, MM > -- > _ | Peter J. Holzer | Story must make more sense than reality. > |_|_) | | > | | | h...@hjp.at | -- Charles Stross, "Creative writing > __/ | http://www.hjp.at/ | challenge!" >
