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"body": "PostgreSQL can only perform partition pruning if the WHERE clause contains a comparison with the partitioning key : one side of the comparison operator must be the plain partitioning key. Burying it inside a function call or other expression won't work. Assuming that journeytime is a timestamp with time zone and your days begin and end according to UTC, your WHERE condition should be: WHERE journeytime >= TIMESTAMP WITH TIME ZONE '2025-05-01 00:00:00 UTC' AND journeytime < TIMESTAMP WITH TIME ZONE '2025-05-01 00:00:00 UTC' + INTERVAL '1 day'",
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"body": "Has many characteristics of being AI generated - which is not allowed on this site.",
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"body": "How about you show us the explain plan so we can see? (But the other answer is probably correct - but will likely get deleted since it looks to be AI generated). Also it appears you want a concrete answer, if so, why ask it as an open-ended question for discussion?",
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"body": "<p>I have a PostgreSQL table partitioned by a datetime field called <em>journeyTime</em>.</p>\n<p>If I have a query where the selection criteria is...</p>\n<pre><code>WHERE date_trunc('day', journeyTime) = '01-May-2025'\n</code></pre>\n<p>I would expect the <em><strong>explain plan</strong></em> to show that the query optimiser has immediately zoomed in on the single partition that all the records for that date are in.</p>\n<p>It doesn't. Instead it does a full table scan on all partitions in the table.</p>\n<p>Why?</p>\n",
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"body": "<p>PostgreSQL can only perform <em>partition pruning</em> if the <code>WHERE</code> clause contains a comparison with the <em>partitioning key</em>: one side of the comparison operator must be the plain partitioning key. Burying it inside a function call or other expression won't work.</p>\n<p>Assuming that <code>journeytime</code> is a <code>timestamp with time zone</code> and your days begin and end according to UTC, your <code>WHERE</code> condition should be:</p>\n<pre><code>WHERE journeytime >= TIMESTAMP WITH TIME ZONE '2025-05-01 00:00:00 UTC'\n AND journeytime < TIMESTAMP WITH TIME ZONE '2025-05-01 00:00:00 UTC' + INTERVAL '1 day'\n</code></pre>\n",
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"body": "<p>Has many characteristics of being AI generated - which is not allowed on this site.</p>\n",
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