Re: Query performance problems with partitioned tables

Andreas Haumer wrote:
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> Hi!
>
> I'm currently experimenting with PostgreSQL 8.2.4 and table
> partitioning in order to improve the performance of an
> application I'm working on.
>
> My application is about managing measurement values (lots of!)
> I have one table "t_mv" which stores all the measurement values.
> A single measurement value has a timestamp and belongs
> to a single time series, so table "t_mv" looks like this:
>
> CREATE TABLE t_mv
> (
> zr integer NOT NULL, -- the time series id
> ts timestamp with time zone NOT NULL, -- the timestamp
> ... -- other attributes of a mv
> )
> WITHOUT OIDS;
>
> ALTER TABLE t_mv
> ADD CONSTRAINT pk_mv_zr_ts PRIMARY KEY (zr, ts);
>
> Each time series defines several other attributes which are common
> to all measurement values of this time series (like sampling location,
> physical parameter, aggregation, cardinality, type, visibility, etc.)
>
> The application should be able to handle several thousand
> different time series and hundreds of millions of measurement
> values, so table t_mv can get quite large.
>
> I have tested installations with up to 70 millions rows in t_mv
> and PostgreSQL can handle that with a quite good performance
> even on non high-end machines (operating system is Linux, btw)
>
> But as I expect installations witch much more rows in t_mv, I
> tried to implement a "partitioned tables" concept using inheritance
> and CHECK constraints, just like it is described in the docs
> (e.g. chapter 5.9 in the current PostgreSQL 8.2.4 documentation)
>
> I split the t_mv table on the timestamp attribute to build
> child tables which hold all measurement values for a single month.
> That way I have several tables called "t_mv_YYYYMM" which all
> inherit from "t_mv". The number of child tables depends on the
> time period the application has to store the measurement values
> (which can be several years so I'm expecting up to 100 child
> tables or even more).
> For the application everything looks the same: inserts, updates
> and queries all are against the "t_mv" parent table, the application
> is not aware of the fact that this table is actually "split" into
> several child tables.
>
> This is working fine and for some standard queries it actually
> gives some performance improvement compared to the standard
> "everything in one big table" concept. The performance improvement
> increases with the number of rows in t_mv, for a small table (less
> than 10 million rows or so) IMHO it is not really worth the effort
> or even counter-productive.
>
> But I have some special queries where the performance with
> partitioned tables actually get much worse: those are queries where
> I'm working with "open" time intervals, i.e. where I want to
> get the previous and/or next timestamp from a given interval.
>
> A simple example: Get the timestamp of a measurement value for time
> series 3622 which is right before the measurement value with time
> stamp '2007-04-22 00:00:00':
>
> testdb_std=> select ts from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' order by ts desc limit 1;
> ts
> - ------------------------
> 2007-04-21 23:00:00+02
> (1 row)
>
>
> Im my application there are many queries like this. Such
> queries also come in several variations, including quite
> sophisticated joins with lots of other tables "above" the
> time series table.
>
> Note: as I'm working with (potentially) non-equidistant
> time series I can not just calculate the timestamps, I
> have to retrieve them from the database!
>
> In the standard case, the query plan for the example query looks like this:
>
> testdb_std=> explain analyze select ts from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' order by ts desc limit 1;
> QUERY PLAN
> - -----------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.00..1.70 rows=1 width=8) (actual time=0.233..0.235 rows=1 loops=1)
> -> Index Scan Backward using pk_mv_zr_ts on t_mv (cost=0.00..21068.91 rows=12399 width=8) (actual time=0.221..0.221 rows=1 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> Total runtime: 0.266 ms
> (4 rows)
>
>
> If I switch to partitioned tables, the query retrieves the same result (of course):
>
> testdb_std=> \c testdb_part
> You are now connected to database "testdb_part".
> testdb_part=> select ts from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' order by ts desc limit 1;
> ts
> - ------------------------
> 2007-04-21 23:00:00+02
> (1 row)
>
>
> But the query plan becomes:
>
> testdb_part=> explain analyze select ts from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' order by ts desc limit 1;
> QUERY PLAN
> - ---------------------------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=23985.83..23985.83 rows=1 width=8) (actual time=230.100..230.102 rows=1 loops=1)
> -> Sort (cost=23985.83..24019.84 rows=13605 width=8) (actual time=230.095..230.095 rows=1 loops=1)
> Sort Key: mwdb.t_mv.ts
> -> Result (cost=0.00..23051.72 rows=13605 width=8) (actual time=0.154..177.519 rows=15810 loops=1)
> -> Append (cost=0.00..23051.72 rows=13605 width=8) (actual time=0.149..114.186 rows=15810 loops=1)
> -> Index Scan using pk_mv_zr_ts on t_mv (cost=0.00..8.27 rows=1 width=8) (actual time=0.047..0.047 rows=0 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200507 on t_mv_200507 t_mv (cost=0.00..2417.53 rows=1519 width=8) (actual time=0.095..2.419 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200508 on t_mv_200508 t_mv (cost=0.00..918.81 rows=539 width=8) (actual time=0.081..2.134 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200509 on t_mv_200509 t_mv (cost=0.00..941.88 rows=555 width=8) (actual time=0.061..2.051 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200510 on t_mv_200510 t_mv (cost=0.00..915.29 rows=538 width=8) (actual time=0.064..2.113 rows=715 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200511 on t_mv_200511 t_mv (cost=0.00..925.93 rows=545 width=8) (actual time=0.048..2.986 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200512 on t_mv_200512 t_mv (cost=0.00..936.53 rows=550 width=8) (actual time=0.049..2.212 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200601 on t_mv_200601 t_mv (cost=0.00..981.42 rows=579 width=8) (actual time=0.065..3.029 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200602 on t_mv_200602 t_mv (cost=0.00..856.25 rows=502 width=8) (actual time=0.045..2.866 rows=672 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200603 on t_mv_200603 t_mv (cost=0.00..977.84 rows=575 width=8) (actual time=0.052..3.044 rows=743 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200604 on t_mv_200604 t_mv (cost=0.00..906.40 rows=531 width=8) (actual time=0.053..1.976 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200605 on t_mv_200605 t_mv (cost=0.00..938.28 rows=550 width=8) (actual time=0.050..2.357 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200606 on t_mv_200606 t_mv (cost=0.00..922.35 rows=541 width=8) (actual time=0.054..2.063 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200607 on t_mv_200607 t_mv (cost=0.00..2112.64 rows=1315 width=8) (actual time=0.047..2.226 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200608 on t_mv_200608 t_mv (cost=0.00..990.23 rows=582 width=8) (actual time=0.048..2.094 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200609 on t_mv_200609 t_mv (cost=0.00..902.84 rows=528 width=8) (actual time=0.039..2.252 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200610 on t_mv_200610 t_mv (cost=0.00..964.87 rows=567 width=8) (actual time=0.033..2.118 rows=745 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200611 on t_mv_200611 t_mv (cost=0.00..947.17 rows=557 width=8) (actual time=0.060..2.160 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200612 on t_mv_200612 t_mv (cost=0.00..929.43 rows=545 width=8) (actual time=0.039..2.051 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200701 on t_mv_200701 t_mv (cost=0.00..940.05 rows=551 width=8) (actual time=0.036..2.217 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200702 on t_mv_200702 t_mv (cost=0.00..847.38 rows=496 width=8) (actual time=0.035..1.830 rows=672 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200703 on t_mv_200703 t_mv (cost=0.00..956.00 rows=561 width=8) (actual time=0.062..2.326 rows=743 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200704 on t_mv_200704 t_mv (cost=0.00..814.38 rows=378 width=8) (actual time=0.050..1.406 rows=504 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> Total runtime: 231.730 ms
> (52 rows)
>
> Oops!
> Compare the costs or the actual query time between those queries!
> (Note: I set "constraint_exclusion = on", of course!)
>
> As such queries are used all over the application, this nullifies
> any performance improvements for standard queries and in fact makes
> the overall application performance as "feeled" by the user _much_
> worse.
>
> I also tried it with "min()" and "max()" aggregate functions
> instead of the "limit 1" query, but this does not change much:
>
>
> Standard "big" table:
>
> testdb_std=> select max(ts) from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' ;
> max
> - ------------------------
> 2007-04-21 23:00:00+02
> (1 row)
>
>
> testdb_std=> explain analyze select max(ts) from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' ;
> QUERY PLAN
> - -------------------------------------------------------------------------------------------------------------------------------------------------
> Result (cost=1.70..1.71 rows=1 width=0) (actual time=0.071..0.073 rows=1 loops=1)
> InitPlan
> -> Limit (cost=0.00..1.70 rows=1 width=8) (actual time=0.060..0.062 rows=1 loops=1)
> -> Index Scan Backward using pk_mv_zr_ts on t_mv (cost=0.00..21068.91 rows=12399 width=8) (actual time=0.056..0.056 rows=1 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> Filter: ((ts)::timestamp with time zone IS NOT NULL)
> Total runtime: 0.221 ms
> (7 rows)
>
>
> "Partitioned table":
>
> testdb_part=> select max(ts) from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' ;
> max
> - ------------------------
> 2007-04-21 23:00:00+02
> (1 row)
>
> testdb_part=> explain analyze select max(ts) from mwdb.t_mv where zr=3622 and ts < '2007-04-22 00:00:00' ;
> QUERY PLAN
> - ----------------------------------------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=23085.73..23085.74 rows=1 width=8) (actual time=390.094..390.096 rows=1 loops=1)
> -> Append (cost=0.00..23051.72 rows=13605 width=8) (actual time=0.241..290.934 rows=15810 loops=1)
> -> Index Scan using pk_mv_zr_ts on t_mv (cost=0.00..8.27 rows=1 width=8) (actual time=0.038..0.038 rows=0 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200507 on t_mv_200507 t_mv (cost=0.00..2417.53 rows=1519 width=8) (actual time=0.197..12.598 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200508 on t_mv_200508 t_mv (cost=0.00..918.81 rows=539 width=8) (actual time=0.095..5.947 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200509 on t_mv_200509 t_mv (cost=0.00..941.88 rows=555 width=8) (actual time=0.118..2.247 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200510 on t_mv_200510 t_mv (cost=0.00..915.29 rows=538 width=8) (actual time=0.121..6.219 rows=715 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200511 on t_mv_200511 t_mv (cost=0.00..925.93 rows=545 width=8) (actual time=2.287..9.991 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200512 on t_mv_200512 t_mv (cost=0.00..936.53 rows=550 width=8) (actual time=0.110..2.285 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200601 on t_mv_200601 t_mv (cost=0.00..981.42 rows=579 width=8) (actual time=0.209..4.682 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200602 on t_mv_200602 t_mv (cost=0.00..856.25 rows=502 width=8) (actual time=0.079..6.079 rows=672 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200603 on t_mv_200603 t_mv (cost=0.00..977.84 rows=575 width=8) (actual time=0.091..4.793 rows=743 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200604 on t_mv_200604 t_mv (cost=0.00..906.40 rows=531 width=8) (actual time=0.108..7.637 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200605 on t_mv_200605 t_mv (cost=0.00..938.28 rows=550 width=8) (actual time=0.116..4.772 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200606 on t_mv_200606 t_mv (cost=0.00..922.35 rows=541 width=8) (actual time=0.074..6.071 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200607 on t_mv_200607 t_mv (cost=0.00..2112.64 rows=1315 width=8) (actual time=0.082..4.807 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200608 on t_mv_200608 t_mv (cost=0.00..990.23 rows=582 width=8) (actual time=2.283..8.671 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200609 on t_mv_200609 t_mv (cost=0.00..902.84 rows=528 width=8) (actual time=0.107..6.067 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200610 on t_mv_200610 t_mv (cost=0.00..964.87 rows=567 width=8) (actual time=0.074..3.933 rows=745 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200611 on t_mv_200611 t_mv (cost=0.00..947.17 rows=557 width=8) (actual time=0.091..6.291 rows=720 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200612 on t_mv_200612 t_mv (cost=0.00..929.43 rows=545 width=8) (actual time=0.077..4.101 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200701 on t_mv_200701 t_mv (cost=0.00..940.05 rows=551 width=8) (actual time=0.077..2.558 rows=744 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200702 on t_mv_200702 t_mv (cost=0.00..847.38 rows=496 width=8) (actual time=0.073..4.346 rows=672 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200703 on t_mv_200703 t_mv (cost=0.00..956.00 rows=561 width=8) (actual time=2.532..7.206 rows=743 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> -> Index Scan using pk_mv_200704 on t_mv_200704 t_mv (cost=0.00..814.38 rows=378 width=8) (actual time=0.120..4.163 rows=504 loops=1)
> Index Cond: (((zr)::integer = 3622) AND ((ts)::timestamp with time zone < '2007-04-22 00:00:00+02'::timestamp with time zone))
> Total runtime: 394.384 ms
> (49 rows)
>
>
> Now my question is: Does the query planner in the case of partitioned tables
> really have to scan all indexes in order to get the next timestamp smaller
> (or larger) than a given one?
>
> There are check conditions on all table partitions like this:
>
> For table t_mv_200704:
> CHECK (ts::timestamp with time zone >= '2007-04-01 00:00:00+02'::timestamp with time zone
> AND ts::timestamp with time zone < '2007-05-01 00:00:00+02'::timestamp with time zone)
>
> For table t_mv_200703:
> CHECK (ts::timestamp with time zone >= '2007-03-01 00:00:00+01'::timestamp with time zone
> AND ts::timestamp with time zone < '2007-04-01 00:00:00+02'::timestamp with time zone)
>
> and so on...
>
> So the tables are in a well defined, monotonic sort order regarding the timestamp.
>
> This means that if there is a max(ts) for ts < '2007-04-22 00:00:00'
> already in table t_mv_200704, it makes no sense to look further in
> other tables where the timestamps can only be smaller than the
> timestamp already found. Am I correct?
>
> Is there room for improvements of the query planner for queries
> like this or is this a special case which will never get handled
> anyway?
>
> Or would you suggest a completely different table structure
> or perhaps some other query?
>
> I'm open for any suggestion!
>
> - - andreas
>
> - --
> Andreas Haumer | mailto:andreas(at)xss(dot)co(dot)at
> *x Software + Systeme | http://www.xss.co.at/
> Karmarschgasse 51/2/20 | Tel: +43-1-6060114-0
> A-1100 Vienna, Austria | Fax: +43-1-6060114-71
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> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
> choose an index scan if your joining column's datatypes do not
> match
>
Hello, Andreas, I too am having exactly the same issue as you do.
Comparing my partitioned and plain table performance, I've found that
the plain tables perform about 25% faster than partitioned table. Using
'explain select ...', I see that constraints are being used so in
partitioned tables fewer rows are examined. But still partitioned tables
are 25% slower, what a let down.

Fei