Re: a JOIN to a VIEW seems slow

Pavel Stehule wrote:

2017-09-14 10:14 GMT+02:00 Frank Millman <frank(at)chagford(dot)com>:

Hi all

This is a follow-up to a recent question I posted regarding a slow query. I thought that the slowness was caused by the number of JOINs in the query, but with your assistance I have found the true reason. I said in the previous thread that the question had become academic, but now that I understand things better, it is no longer academic as it casts doubt on my whole approach.

I have split my AR transaction table into three physical tables – ar_tran_inv, ar_tran_crn, ar_tran_rec. I will probably add others at some point, such as ar_tran_jnl.

I then create a VIEW to view all transactions combined. The view is created like this -

CREATE VIEW ar_trans AS
SELECT ‘ar_inv’ AS tran_type, row_id AS tran_row_id, tran_number ... FROM ar_tran_inv WHERE posted = ‘1’
UNION ALL
SELECT ‘ar_crn’ AS tran_type, row_id AS tran_row_id, tran_number ... FROM ar_tran_crn WHERE posted = ‘1’
UNION ALL
SELECT ‘ar_rec’ AS tran_type, row_id AS tran_row_id, tran_number ... FROM ar_tran_rec WHERE posted = ‘1’

I have another table called ‘ar_trans_due’, to keep track of outstanding transactions. All of the three transaction types generate entries into this table. To identify the source of the transaction, I have created columns in ar_trans_due called ‘tran_type’ and ‘tran_row_id’. After inserting a row into ‘ar_tran_inv’, I invoke this -

INSERT INTO ar_trans_due (tran_type, tran_row_id, ...) VALUES (‘ar_inv’, ar_tran_inv.row_id, ...), and similar for the other transaction types. It is handled by a Python program, and it all happens within a transaction.

When I view a row in ar_trans_due, I want to retrieve data from the source transaction, so I have this -

SELECT * FROM ar_trans_due a
LEFT JOIN ar_trans b ON b.tran_type = a.tran_type AND b.tran_row_id = a.tran_row_id

I understand that PostgreSQL must somehow follow a path from the view ‘ar_trans’ to the physical table ‘ar_tran_inv’, but I assumed it would execute the equivalent of SELECT * FROM ar_tran_inv WHERE row_id = a.tran_row_id AND posted = ‘1’.

If this was the case, it would be an indexed read, and very fast. Instead, according to EXPLAIN, it performs a sequential scan of the ‘ar_tran_inv’ table.

It also scans ‘ar_tran_crn’ and ‘ar_tran_rec’, but EXPLAIN shows that it uses a Bitmap Heap Scan on those. I assume that is because the tables are currently empty.

Is this analysis correct?

please, send EXPLAIN ANALYZE result :)

I tried to reduce this to its simplest form.

Here is a SQL statement -

SELECT *
FROM ccc.ar_trans_due a
LEFT JOIN ccc.ar_trans b ON
b.tran_type = a.tran_type AND b.tran_row_id = a.tran_row_id
WHERE a.row_id = 1

ar_trans_due is a physical table, ar_trans is a view.

It takes about 28ms. Here is the explain - https://explain.depesz.com/s/8YY

Then I changed it to join each of the physical tables, instead of the view -

SELECT *
FROM ccc.ar_trans_due a
LEFT JOIN ccc.ar_tran_inv b ON
b.row_id = a.tran_row_id
LEFT JOIN ccc.ar_tran_crn c ON
c.row_id = a.tran_row_id
LEFT JOIN ccc.ar_tran_rec d ON
d.row_id = a.tran_row_id
WHERE a.row_id = 1

This takes just over 1ms. Here is the explain - https://explain.depesz.com/s/U29h

I tried setting enable_seq_scan to off – it ran even slower!

Frank