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functional_limitations.md

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Functional limitations

Influences of some of planner GUC parameters

The planner does not try to consider joining order for FROM clause entries more than from_collapse_limit. pg_hint_plan cannot affect joining order as expected for the case.

Hints trying to enforce unexecutable plans

Planner chooses any executable plans when the enforced plan cannot be executed.

  • FULL OUTER JOIN to use nested loop
  • To use indexes that does not have columns used in quals
  • To do TID scans for queries without ctid conditions

Queries in ECPG

ECPG removes comments in queries written as embedded SQLs so hints cannot be passed form those queries. The only exception is that EXECUTE command passes given string unmodifed. Please consider using the hint table in the case.

Work with pg_stat_statements

pg_stat_statements generates a query id ignoring comments. As the result, the identical queries with different hints are summarized as the same query.