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POPostgreSQL Query Optimization and the Postmaster Process'
 

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    <p>I currently working with a larger wikipedia-dump derived PostgreSQL database; it contains about 40 GB of data. The database is running on an HP Proliant ML370 G5 server with Suse Linux Enterprise Server 10; I am querying it from my laptop over a private network managed by a simple D-Link router. I assigned static DHCP (private) IPs to both laptop and server.</p> <p>Anyway, from my laptop, using pgAdmin III, I send off some SQL commands/queries; some of these are CREATE INDEX, DROP INDEX, DELETE, SELECT, etc. Sometimes I send a command (like CREATE INDEX), it returns, telling me that the query was executed perfectly, etc. However, the postmaster process assigned to such a command seems to remain sleeping on the server. Now, I do not really mind this, for I say to myself that PostgreSQL maintains a pool of postmasters ready to process queries. Yet, if this process eats up 6 GB of it 9.4 GB assigned RAM, I worry (and it does so for the moment). Now maybe this is a cache of data that is kept in [shared] memory in case another query happens to need to use that same data, but I do not know.</p> <p>Another thing is bothering me. </p> <p>I have 2 tables. One is the <em>page</em> table; I have an index on its <em>page_id</em> column. The other is the <em>pagelinks</em> tables which has the <em>pl_from</em> column that references either nothing or a variable in the <em>page.page_id</em> column; unlike the <em>page_id</em> column, the <em>pl_from</em> has no index (yet). To give you an idea of the scale of the tables and the necessity for me to find a viable solution, <em>page</em> table has 13.4 million rows (after I deleted those I do not need) while the <em>pagelinks</em> table has 293 million.</p> <p>I need to execute the following command to clean the <em>pagelinks</em> table of some of its useless rows:</p> <pre><code>DELETE FROM pagelinks USING page WHERE pl_from NOT IN (page_id); </code></pre> <p>So basically, I wish to rid the <em>pagelinks</em> table of all links coming from a page not in the <em>page</em> table. Even after disabling the nested loops and/or sequential scans, the query optimizer always gives me the following "solution":</p> <pre><code>Nested Loop (cost=494640.60..112115531252189.59 rows=3953377028232000 width=6) Join Filter: ("outer".pl_from &lt;&gt; "inner".page_id)" -&gt; Seq Scan on pagelinks (cost=0.00..5889791.00 rows=293392800 width=17) -&gt; Materialize (cost=494640.60..708341.51 rows=13474691 width=11) -&gt; Seq Scan on page (cost=0.00..402211.91 rows=13474691 width=11) </code></pre> <p>It seems that such a task would take more than weeks to complete; obviously, this is unacceptable. It seems to me that I would much rather it use the <em>page_id</em> index to do its thing...but it is a stubborn optimizer and I might be wrong. </p>
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