Estimating row count from explain output…in Perl!

At the end of his blog post, Laurenz Albe shows how to use a quick and dirty function to estimate the number of rows returned by an arbitrary query.

While I don’t believe it is often a good idea to judge the size of a query by the optimizer guesses, the approach is interesting. Laurenz shows how to exploit the JSON format and query facilities to extract data from the EXPLAIN output, why not using Perl to crunch the textual data? So here it is a simple implementation to extract the estimate within Perl:
CREATE OR REPLACE FUNCTION plperl_row_estimate( query text )

   my ( $query ) = @_;
   return 0 if ( ! $query );
   $query = sprintf "EXPLAIN (FORMAT YAML) %s", $query;

   elog( DEBUG, "Estimating from [$query]" );
   my @estimated_rows = map { s/Plan Rows:\s+(\d+)$/$1/; $_ }
                        grep { $_ =~ /Plan Rows:/ }
                        split( "\n", spi_exec_query( $query )->{ rows }[ 0 ]->{ "QUERY PLAN" } );

   return 0 if ( ! @estimated_rows );
   return $estimated_rows[ 0 ];
LANGUAGE plperl;
Let’s see an example in action:
testdb=> select plperl_row_estimate( 'SELECT p.* FROM persona p JOIN persona k on = WHERE k.eta = 40' );

How does the function works? The main trick is at this point in code:
   my @estimated_rows = map { s/Plan Rows:\s+(\d+)$/$1/; $_ }
                        grep { $_ =~ /Plan Rows:/ }
                        split( "\n", spi_exec_query( $query )->{ rows }[ 0 ]->{ "QUERY PLAN" } );

where thru spi_exec_query an EXPLAIN is executed and its format, in YAML is split into an array of strings, one entry per line. Such array, is then passed to grep to exclude all rows that do not contain information about the row estimation. Last, map extracts the numeric value from such lines.

After that, therefore, there is an array of @estimated_rows entries where each one contains the rows estimatation of each plan node, with the outer node in the begin of the array. Such single position is therefore returned by the function and all the others are dropped away.

As a final note, please consider that such function accepts an arbitrary piece of text and tries to execute it as a query, therefore it must be used carefully to avoid SQL-injection and problems alike.

The article Estimating row count from explain Perl! has been posted by Luca Ferrari on April 4, 2019