Improve performance of EXPLAIN with large range tables

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 15 January 2015 12:18 UTC

As of 9.3, ruleutils.c goes to some lengths to ensure that table and column aliases used in its output are unique. Of course this takes more time than was required before, which in itself isn't fatal. However, EXPLAIN was set up so that recalculation of the unique aliases was repeated for each subexpression printed in a plan. That results in O(N^2) time and memory consumption for large plan trees, which did not happen in older branches.

Fortunately, the expensive work is the same across a whole plan tree, so there is no need to repeat it; we can do most of the initialization just once per query and re-use it for each subexpression. This buys back most (not all) of the performance loss since 9.2.

We need an extra ExplainState field to hold the precalculated deparse context. That's no problem in HEAD, but in the back branches, expanding sizeof(ExplainState) seems risky because third-party extensions might have local variables of that struct type. So, in 9.4 and 9.3, introduce an auxiliary struct to keep sizeof(ExplainState) the same. We should refactor the APIs to avoid such local variables in future, but that's material for a separate HEAD-only commit.

Per gripe from Alexey Bashtanov. Back-patch to 9.3 where the issue was introduced.

a5cd70d Improve performance of EXPLAIN with large range tables.
src/backend/commands/explain.c | 23 ++++++------
src/backend/utils/adt/ruleutils.c | 73 +++++++++++++++++++++++++------------
src/include/commands/explain.h | 1 +
src/include/utils/ruleutils.h | 7 ++--
4 files changed, 66 insertions(+), 38 deletions(-)

Upstream: git.postgresql.org

  • Share