Join Multiplication

Candidate Pair Grid

Every left row is compared with every right row.

highlighted = computed this step

Candidate pair grid

The finite join compares every left row with every right row.

left times right\text{left times right}

Count pairs

The compiler checks 20 candidate pairs in left-major order.

pairs=20\text{pairs}=20

Inner-join examples are tiny finite-table transforms; SQL dialect completeness, type coercion, optimizer behavior, execution cost, indexing, and database-product claims are out of scope.

Candidate pair grid: left tablecustomer_idname1Ada2Ben3CyNULLNoKey Candidate pair grid: right tablecustomer_idorder_iditem1101book1102pen2103bag4104lampNULL105mystery candidate row pairsleftSourcerightSourceleftKeyrightKeymatched00[1][1]yes01[1][1]yes02[1][2]no03[1][4]no04[1][NULL]no10[2][1]no11[2][1]no12[2][2]yes13[2][4]no14[2][NULL]no20[3][1]no21[3][1]no22[3][2]no23[3][4]no24[3][NULL]no30[NULL][1]no31[NULL][1]no32[NULL][2]no33[NULL][4]no34[NULL][NULL]no inner join rowsleft.customer_idleft.nameright.customer_idright.order_idright.item1Ada1101book1Ada1102pen2Ben2103bag inner join factsfactvalueleftRowCount4rightRowCount5candidatePairCount20matchingPairCount3unmatchedLeftRows2unmatchedRightRows2joinRowCount3keyColumnCount1nullKeyMatches0rowOrderleft_source_then_right_source

Read pair order

Pair order is left source row first, then right source row.

deterministic order\text{deterministic order}

Summary

Candidate pairs are possible rows, not output rows yet.

possible rows\text{possible rows}