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5.8. Patterns<br />
5.8.1 Checking the Number of Rows<br />
74. Never use SELECT COUNT(*) if you are only interested in the existence of a row.<br />
[CodeXpert 5804]<br />
Reason: If you do a SELECT count(*) all rows will be read according to the WHERE<br />
clause, even if only the availability of data is of interest. For this we have a<br />
big performance overhead. If we do a SELECT count(*) .. WHERE<br />
ROWNUM = 1 there is also a overhead as there will be two<br />
communications between the PL/SQL and the SQL engine. See the<br />
following example for a better solution.<br />
Example:<br />
-- Bad<br />
...<br />
BEGIN<br />
SELECT count(*)<br />
INTO l_count<br />
FROM cust_order<br />
WHERE …<br />
IF l_count > 0<br />
THEN<br />
SELECT p.part_nbr, p.name, p.unit_cost<br />
FROM part p<br />
WHERE …<br />
-- Good<br />
...<br />
BEGIN<br />
SELECT p.part_nbr, p.name, p.unit_cost<br />
FROM part p<br />
WHERE EXISTS (SELECT 1<br />
FROM cust_order co<br />
WHERE co.part_nbr = p.part_nbr)<br />
…<br />
PL/SQL Coding Guidelines 43
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