Our current docs have this text for PREPARE:
Prepared statements can use generic plans rather than re-planning
with each set of supplied EXECUTE values. This occurs immediately
for prepared statements with no parameters; otherwise it occurs
only after five or more executions produce plans whose estimated
cost average (including planning overhead) is more expensive than
the generic plan cost estimate. Once a generic plan is chosen, it
is used for the remaining lifetime of the prepared statement. Using
EXECUTE values which are rare in columns with many duplicates can
generate custom plans that are so much cheaper than the generic
plan, even after adding planning overhead, that the generic plan
might never be used.
There is no mention that PG 12's plan_cache_mode can modify this
behavior. I think this needs a doc patch.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
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