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    <p>I always try to put the expressions in <code>JOIN</code>'s in the following order:</p> <pre><code>joined.column = leading.column </code></pre> <p>This query:</p> <pre><code>SELECT t.value, d.name FROM transactions t JOIN dimensions d ON d.id = t.dimension </code></pre> <p>can be treated either like "for each transaction, find the corresponding dimension name", or "for each dimension, find all corresponding transaction values".</p> <p>So, if I search for a given transaction, I put the expressions in the following order:</p> <pre><code>SELECT t.value, d.name FROM transactions t JOIN dimensions d ON d.id = t.dimension WHERE t.id = :myid </code></pre> <p>, and if I search for a dimension, then:</p> <pre><code>SELECT t.value, d.name FROM dimensions d JOIN transactions t ON t.dimension = d.id WHERE d.id = :otherid </code></pre> <p>Ther former query will most probably use index scans first on <code>(t.id)</code>, then on (<code>d.id</code>), while the latter one will use index scans first on <code>(d.id)</code>, then on <code>(t.dimension)</code>, and you can easily see it in the query itself: the searched fields are at left.</p> <p>The driving and driven tables may be not so obvious in a <code>JOIN</code>, but it's as clear as a bell for a <code>CONNECT BY</code> query: the <code>PRIOR</code> row is driving, the non-<code>PRIOR</code> is driven.</p> <p>That's why this query:</p> <pre><code>SELECT * FROM hierarchy START WITH id = :root CONNECT BY parent = PRIOR id </code></pre> <p>means "find all rows whose <code>parent</code> is a given <code>id</code>". This query builds a hierarchy.</p> <p>This can be treated like this:</p> <pre><code>connect_by(row) { add_to_rowset(row); /* parent = PRIOR id */ /* PRIOR id is an rvalue */ index_on_parent.searchKey = row-&gt;id; foreach child_row in index_on_parent.search { connect_by(child_row); } } </code></pre> <p>And this query:</p> <pre><code>SELECT * FROM hierarchy START WITH id = :leaf CONNECT BY id = PRIOR parent </code></pre> <p>means "find the rows whose <code>id</code> is a given <code>parent</code>". This query builds an ancestry chain.</p> <p><strong>Always put <code>PRIOR</code> in the right part of the expression.</strong></p> <p><strong>Think of <code>PRIOR column</code> as of a constant all your rows will be searched for.</strong></p>
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    1. POHow do I remember which way round PRIOR should go in CONNECT BY queries
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    1. USQuassnoi
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    1. COI like "Always put PRIOR in the right part of the expression" - it does make it easier. Also putting "start with" at the start.
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      1. USNick Pierpoint
 

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