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PODeadlock involving foreign key constraint
 

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    <p>I would like to understand better a mechanism of locking in postgres.</p> <p>Let's say that tree can have apples (via foreign key on apple table). It seems that when selecting a tree for update lock is obtained on an apple. However, the operation is not blocked even if someone else already holds a lock on this apple.</p> <p>Why is it so?</p> <p>p.s. Please don't suggest to remove "select for update".</p> <h1>Scenario</h1> <pre><code>Transaction 1 Transaction 2 BEGIN . update apple; . . BEGIN . select tree for update; . update apple; . --halts because of the other transaction locking an apple update apple; . -- deadlock . COMMIT --transaction succeeds </code></pre> <h1>Code</h1> <p>If you want to try it in your postgres - here is a code you can copy/paste.</p> <p>I have a following db schema</p> <pre><code>CREATE TABLE trees ( id integer primary key ); create table apples ( id integer primary key, tree_id integer references trees(id) ); </code></pre> <p>and very simple data</p> <pre><code>insert into trees values(1); insert into apples values(1,1); </code></pre> <p>There are two simple transactions. One is updating apples, the second is locking a tree and updating an apple.</p> <pre><code>BEGIN; UPDATE apples SET id = id WHERE id = 1; -- run second transaction in paralell UPDATE apples SET id = id WHERE id = 1; COMMIT; BEGIN; SELECT id FROM trees WHERE id = 1 FOR UPDATE; UPDATE apples SET id = id WHERE id = 1; COMMIT; </code></pre> <p>When I run them - deadlock occurs on the second update of first transaction.</p> <pre><code>ERROR: deadlock detected DETAIL: Process 81122 waits for ShareLock on transaction 227154; blocked by process 81100. Process 81100 waits for ShareLock on transaction 227153; blocked by process 81122. CONTEXT: SQL statement "SELECT 1 FROM ONLY "public"."trees" x WHERE "id" OPERATOR(pg_catalog.=) $1 FOR SHARE OF x" </code></pre>
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    1. USKuba
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    1. COWhich version is this? It doesn't look like `ShareLock` is used in the latest version. Is the `UPDATE` necessary in the second transaction?
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      1. USJohn Tseng
 

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