From 36a4c6248bd4eec9062ed477b7c3f551738e3c96 Mon Sep 17 00:00:00 2001
From: junhangis <xieweiping@xfusion.com>
Date: Sun, 7 Aug 2022 02:11:01 +0800
Subject: [PATCH] improve the performance of numeric subtraction.
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before optimization£º
openGauss=# explain analyse select t0 - t1 from t_numeric limit 10000000;
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..292129.20 rows=8468656 width=10) (actual time=4.688..11066.444 rows=8388608 loops=1)
   ->  Seq Scan on t_numeric  (cost=0.00..292129.20 rows=8468656 width=10) (actual time=4.686..9901.704 rows=8388608 loops=1)
 Total runtime: 12103.123 ms
(3 rows)

Time: 12106.419 ms

after optimization£º
openGauss=# explain analyse select t0 - t1 from t_numeric limit 10000000;
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..292129.20 rows=8468656 width=10) (actual time=4.182..9914.114 rows=8388608 loops=1)
   ->  Seq Scan on t_numeric  (cost=0.00..292129.20 rows=8468656 width=10) (actual time=4.180..8732.662 rows=8388608 loops=1)
 Total runtime: 10918.849 ms
(3 rows)

Time: 10919.690 ms
---
 src/common/backend/utils/adt/numeric.cpp | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/common/backend/utils/adt/numeric.cpp b/src/common/backend/utils/adt/numeric.cpp
index 59a6890be..58c9cb255 100644
--- a/src/common/backend/utils/adt/numeric.cpp
+++ b/src/common/backend/utils/adt/numeric.cpp
@@ -2174,7 +2174,7 @@ Datum numeric_sub(PG_FUNCTION_ARGS)
     init_var_from_num(num1, &arg1);
     init_var_from_num(num2, &arg2);
 
-    init_var(&result);
+    quick_init_var(&result);
     sub_var(&arg1, &arg2, &result);
 
     res = make_result(&result);
@@ -6705,6 +6705,7 @@ static void sub_abs(NumericVar* var1, NumericVar* var2, NumericVar* result)
     int var2ndigits = var2->ndigits;
     NumericDigit* var1digits = var1->digits;
     NumericDigit* var2digits = var2->digits;
+    NumericDigit tdig[NUMERIC_LOCAL_NDIG];
 
     res_weight = var1->weight;
 
@@ -6720,7 +6721,10 @@ static void sub_abs(NumericVar* var1, NumericVar* var2, NumericVar* result)
         res_ndigits = 1;
     }
 
-    res_buf = digitbuf_alloc(res_ndigits + 1);
+    res_buf = tdig;
+    if (res_ndigits > NUMERIC_LOCAL_NMAX) {
+        res_buf = digitbuf_alloc(res_ndigits + 1);
+    }
     res_buf[0] = 0; /* spare digit for later rounding */
     res_digits = res_buf + 1;
 
@@ -6747,8 +6751,18 @@ static void sub_abs(NumericVar* var1, NumericVar* var2, NumericVar* result)
 
     digitbuf_free(result);
     result->ndigits = res_ndigits;
-    result->buf = res_buf;
-    result->digits = res_digits;
+    if (res_buf != tdig)
+    {
+        result->buf = res_buf;
+        result->digits = res_digits;
+    } else {
+        result->buf = result->ndb;
+        result->digits = result->buf;
+        errno_t rc = memcpy_s(result->buf, sizeof(NumericDigit) * (res_ndigits + 1),
+                              res_buf, sizeof(NumericDigit) * (res_ndigits + 1));
+        securec_check(rc, "\0", "\0");
+        result->digits ++;
+    }
     result->weight = res_weight;
     result->dscale = res_dscale;