Re: Different query plan used for the same query depending on how parameters are passed

[David Chapman <david.chapman@xxxxxxxxxxxxxxxxxxx>]

Version is 'PostgreSQL 9.5.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4), 64-bit'

Here is the output of EXPLAIN ANALYZE on the two queries.

Index Scan using test_index_t1_t2 on test  (cost=0.43..684.11 rows=71 width=245) (actual time=0.022..1.147 rows=99 loops=1)

  Index Cond: ((t1 = 'X'::bpchar) AND (t2 = ANY ('{2286575,2139022,2139030,2139032,1912037,860952,2139035,2139037,2278996,2139043,72062644919750111,2139045,2139047,1904847,2139049,2259635,2259633,2293287,2293281,2206950,2127033,2206952,2146439,2139064,2139066,2139070,2139074,2305447,2139077,2377493,2139079,2282669,2139083,2375729,2260985,2286579,2274454,2286583,2286582,2281512,2286588,2286591,2286592,1912217,2286596,2307845,2307846,2293294,287038,2293295,2293291,2356192,2317356,2356191,2356190,2356181,2356195,2356197,2389077,2356199,2356217,2374842,2374844,2374846,2337988,2374856,1950450,2374833,2374839,2282877,2374843,2374849,2374855,2390631,2028018,2374834,2139011,2139004,2344765,2344767,2390620,2393192,2344771,2344773,2344775,2390201,2344777,1893069,2344779,2344781,2344783,2374838,2304999,2344762,2344764,2344766,2344768,2344772,867138,2276706}'::text[])))

Planning time: 0.779 ms

Execution time: 1.417 ms

Seq Scan on test  (cost=0.00..403725.30 rows=1 width=245) (actual time=47.543..5362.518 rows=99 loops=1)

  Filter: (((t1)::text = 'X'::text) AND (t2 = ANY ('{2286575,2139022,2139030,2139032,1912037,860952,2139035,2139037,2278996,2139043,72062644919750111,2139045,2139047,1904847,2139049,2259635,2259633,2293287,2293281,2206950,2127033,2206952,2146439,2139064,2139066,2139070,2139074,2305447,2139077,2377493,2139079,2282669,2139083,2375729,2260985,2286579,2274454,2286583,2286582,2281512,2286588,2286591,2286592,1912217,2286596,2307845,2307846,2293294,287038,2293295,2293291,2356192,2317356,2356191,2356190,2356181,2356195,2356197,2389077,2356199,2356217,2374842,2374844,2374846,2337988,2374856,1950450,2374833,2374839,2282877,2374843,2374849,2374855,2390631,2028018,2374834,2139011,2139004,2344765,2344767,2390620,2393192,2344771,2344773,2344775,2390201,2344777,1893069,2344779,2344781,2344783,2374838,2304999,2344762,2344764,2344766,2344768,2344772,867138,2276706}'::text[])))

  Rows Removed by Filter: 2327145

Planning time: 0.179 ms

Execution time: 5362.558 ms

As it turns out t1 was actually defined as character(1) rather than text. I can see Postgres has cast the first parameter to bpchar in the first case but I guess Npgsql is explicitly sending it as type text, which bypasses the index.

On 16 May 2017 at 15:17, Adrian Klaver <adrian.klaver@xxxxxxxxxxx> wrote:

On 05/16/2017 06:01 AM, David Chapman wrote:

I have a table that includes two text columns t1 and t2, and a composite index on these columns. When issuing a query of the following form:

SELECT * FROM test WHERE t1 = 'X' and t2 = ANY(ARRAY['Y1', 'Y2', ..])

I have observed that it will use the index and have reasonable performance if the whole query is passed as a single big string. However if it is parameterised (I'm using Npgsql) it switches to doing a sequence scan and performs terribly.

What Postgres version?

Can you show the parametrized version?

What is the output if you add EXPLAIN ANALYZE to the beginning of the query?:

https://www.postgresql.org/docs/9.6/static/sql-explain.html

The table contains approx 2.3 million records and the query matches about 20k records (i.e. there are 20k 'Y' values in the array).

I have experimented with changing work_mem, preparing the statement in advance, ANALYZEing the table, none of these change the behavior.

Why does the query planner choose to ignore the index when the command is parameterised?

--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx

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