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PostgreSQL uses shared memory, it doesn't copy the hash table.

• anarazel 9y ago
  In the specific case of hashjoins, it does build them independently right now. There's a patch to rectify that though, by putting the hashtable also into shared memory. The coordination necessary to make multi phase batch joins and other such funny bits work, unfortunately made it infeasible to get into 10.
  • halayli 9y ago
    I stand corrected, it definitely reconstructs the hash table in each process.
    • anarazel 9y ago
      FWIW, here's the patchset to fix that: https://commitfest.postgresql.org/15/871/ https://commitfest.postgresql.org/15/871/
      • halayli 9y ago
        Thanks! I remember reading this thread a while back and I thought it made it in.