PostgreSQL : inconsistent result when using WHERE clause on VIEW

With a bit of trial and error, I was able to spot the culprit. Turns out, it has nothing to do with the view itself. Attempting to manually push the where clause directly into the query also gave the same bad results.

As it turns out, the problem was cause by the CONNECT BY clause in the inner queries. My suspicion (which may be completely wrong) is that it has to do with the fact that Oracle 10g cannot handle recursive CTE's. If anyone has any more information about why this might have happened, I am all ears.

I was able to work around the issue by moving the

SELECT level AS UNPIVOT_ROW FROM DUAL CONNECT BY level <= 3

subquery to its own CTE block and selecting from that as normal. Below I have included the (condensed) working version of the above broken query.

CREATE OR REPLACE VIEW FSA.FSA_V_DB_TOTALS_2
(...)
AS
WITH
--Queries to generate intermediate result sets

--'NUMBERS' CTE block to work around Oracle 10g limitation wherein WHERE
--  clause is ignored if CONNECY BY is directly written into inner queries.
--  NOTE: the size limitation (currently 10) only needs to be larger than
--        or equal to the largest value needed.
NUMBERS AS (
SELECT level AS UNPIVOT_ROW
  FROM DUAL CONNECT BY level <=10
),

--Distribution TRA
DIST_TRA AS (
SELECT ...
  FROM (
       SELECT ...
         FROM FSA.FSA001_DISTRIBUT T1
              INNER JOIN
              FSA.FSA002_DIST_TRA T2 ON T1.SERIAL_NO = T2.SERIAL_NO
        GROUP BY GROUPING SETS (...)
        ),(
        SELECT UNPIVOT_ROW FROM NUMBERS WHERE UNPIVOT_ROW <= 3      -- <==  THIS
        )
),

--Distribution Fran
DIST_FRAN AS (
SELECT ...
  FROM (
       SELECT ...
         FROM FSA.FSA001_DISTRIBUT T1
              INNER JOIN
              FSA.FSA003_DIST_FRAN T3 ON T1.SERIAL_NO = T3.SERIAL_NO
        GROUP BY GROUPING SETS (...)
        ),(
        SELECT UNPIVOT_ROW FROM NUMBERS WHERE UNPIVOT_ROW <= 2      -- <==  THIS AS WELL
        )
)

--Queries to generate final result set based on CTE intermediate queries
--Subtotals / System totals for Dist-TRA, Dist-Fran, Trans-TRA, Trans-Fran
SELECT ...
  FROM (SELECT * FROM DIST_TRA
        UNION ALL
        SELECT * FROM DIST_FRAN)

Consistent rows

The important question which does not seem to be on your radar yet:
From each set of rows for the same seriesName, do you want the columns of one row, or just any values from multiple rows (which may or may not go together)?

Your answer does the latter, you combine the maximum dbid with the maximum retreivaltime, which may come from a different row.

To get consistent rows, use DISTINCT ON and wrap it in a subquery to order the result differently:

SELECT * FROM (
   SELECT DISTINCT ON (seriesName)
          dbid, seriesName, retreivaltime
   FROM   FileItems
   WHERE  sourceSite = 'mk' 
   ORDER  BY seriesName, retreivaltime DESC NULLS LAST  -- latest retreivaltime
   ) sub
ORDER BY retreivaltime DESC NULLS LAST
LIMIT  100;

Details for DISTINCT ON:

• Select first row in each GROUP BY group?

Aside: should probably be retrievalTime, or better yet: retrieval_time. Unquoted mixed case identifiers are a common source of confusion in Postgres.

Better Performance with rCTE

Since we are dealing with a big table here, we'd need a query that can use an index, which is not the case for the above query (except for WHERE sourceSite = 'mk')

On closer inspection, your problem seems to be a special case of a loose index scan. Postgres does not support loose index scans natively, but it can be emulated with a recursive CTE. There is a code example for the simple case in the Postgres Wiki.

Related answer on SO with more advanced solutions, explanation, fiddle:

• Optimize GROUP BY query to retrieve latest record per user

Your case is more complex, though. But I think I found a variant to make it work for you. Building on this index (without WHERE sourceSite = 'mk')

CREATE INDEX mi_special_full_idx ON MangaItems
(retreivaltime DESC NULLS LAST, seriesName DESC NULLS LAST, dbid)

Or (with WHERE sourceSite = 'mk')

CREATE INDEX mi_special_granulated_idx ON MangaItems
(sourceSite, retreivaltime DESC NULLS LAST, seriesName DESC NULLS LAST, dbid)

The first index can be used for both queries, but is not fully efficient with the additional WHERE condition. The second index is of very limited use for the first query. Since you have both variants of the query consider creating both indexes.

I added dbid at the end to allow Index Only scans.

This query with a recursive CTE makes use of the index. I tested with Postgres 9.3 and it works for me: no sequential scan, all index-only scans:

WITH RECURSIVE cte AS (
   (
   SELECT dbid, seriesName, retreivaltime, 1 AS rn, ARRAY[seriesName] AS arr
   FROM   MangaItems
   WHERE  sourceSite = 'mk'
   ORDER  BY retreivaltime DESC NULLS LAST, seriesName DESC NULLS LAST
   LIMIT  1
   )
   UNION ALL
   SELECT i.dbid, i.seriesName, i.retreivaltime, c.rn + 1, c.arr || i.seriesName
   FROM   cte c
   ,      LATERAL (
      SELECT dbid, seriesName, retreivaltime
      FROM   MangaItems
      WHERE (retreivaltime, seriesName) < (c.retreivaltime, c.seriesName)
      AND    sourceSite = 'mk'  -- repeat condition!
      AND    seriesName <> ALL(c.arr)
      ORDER  BY retreivaltime DESC NULLS LAST, seriesName DESC NULLS LAST
      LIMIT  1
      ) i
   WHERE  c.rn < 101
   )
SELECT dbid
FROM   cte
ORDER  BY rn;

You need to include seriesName in ORDER BY, since retreivaltime is not unique. "Almost" unique is still not unique.

Explain

• The non-recursive query starts with the latest row.

• The recursive query adds the next-latest row with a seriesName that's not in the list, yet etc., until we have 100 rows.

• Essential parts are the JOIN condition (b.retreivaltime, b.seriesName) < (c.retreivaltime, c.seriesName) and the ORDER BY clause ORDER BY retreivaltime DESC NULLS LAST, seriesName DESC NULLS LAST. Both match the sort order of the index, which allows for the magic to happen.

• Collecting seriesName in an array to rule out duplicates. The cost for b.seriesName <> ALL(c.foo_arr) grows progressively with the number of rows, but for just 100 rows it is still cheap.

• Just returning dbid as clarified in the comments.

Alternative with partial indexes:

We have been dealing with similar problems before. Here is a highly optimized complete solution based on partial indexes and a looping function:

• Can spatial index help a "range - order by - limit" query

Probably the fastest way (except for a materialized view) if done right. But more complex.

Materialized View

Since you do not have a lot of write operations and they are not performance-critical as stated in the comments (should be in the question), save the top n pre-computed rows in a materialized view and refresh it after relevant changes to the underlying table. Base your performance-critical queries on the materialized view instead.

• Could just be a "thin" mv of the latest 1000 dbid or so. In the query, join to the original table. For instance, if content is sometimes updated, but the top n rows can remain unchanged.

• Or a "fat" mv with whole rows to return. Faster, yet. Needs to be refreshed more often, obviously.

Details in the manual here and here.