Re: ask for review of MERGE

Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> rhaas=# create table concurrent (x integer primary key);
> NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
> "concurrent_pkey" for table "concurrent"
> CREATE TABLE
> rhaas=# insert into x values (1);
> rhaas=# begin;
> BEGIN
> rhaas=# insert into concurrent values (2);
> INSERT 0 1
>
> <switch to a different window>
>
> rhaas=# update concurrent set x=x where x=2;
> UPDATE 0

That surprised me. I would have thought that the INSERT would have
created an "in doubt" tuple which would block the UPDATE. What is
the reason for not doing so?

FWIW I did a quick test and REPEATABLE READ also lets this pass but
with the SSI patch SERIALIZABLE seems to cover this correctly,
generating a serialization failure where such access is involved in
write skew:

test=# create table concurrent (x integer primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"concurrent_pkey" for table "concurrent"
CREATE TABLE
test=# insert into concurrent select generate_series(1, 20000);
INSERT 0 20000
test=# begin isolation level serializable;
BEGIN
test=# insert into concurrent values (0);
INSERT 0 1
test=# update concurrent set x = 30001 where x = 30000;
UPDATE 0

<different session>

test=# begin isolation level serializable;
BEGIN
test=# insert into concurrent values (30000);
INSERT 0 1
test=# update concurrent set x = -1 where x = 0;
UPDATE 0
test=# commit;
ERROR: could not serialize access due to read/write dependencies
among transactions
HINT: The transaction might succeed if retried.

I'll need to add a test to cover this, because it might have broken
with one of the optimizations on my list, had you not point out this
behavior.

On the other hand:

<session 1>

test=# drop table concurrent ;
DROP TABLE
test=# create table concurrent (x integer primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"concurrent_pkey" for table "concurrent"
CREATE TABLE
test=# insert into concurrent select generate_series(1, 20000);
INSERT 0 20000
test=# begin isolation level serializable;
BEGIN
test=# insert into concurrent values (0);
INSERT 0 1

<session 2>

test=# begin isolation level serializable;
BEGIN
test=# select * from concurrent where x = 0;
x
---
(0 rows)

test=# insert into concurrent values (0);
<blocks>

<session 1>

test=# commit;
COMMIT

<session 2>

ERROR: duplicate key value violates unique constraint
"concurrent_pkey"
DETAIL: Key (x)=(0) already exists.

Anyway, I thought this might be of interest in terms of the MERGE
patch concurrency issues, since the SSI patch has been mentioned.

-Kevin